Number

Lemur

mammals

If the animals were food deprived prior to being placed in the experimental condition, the task might become more salient and the results clearer.

"If the numerical abilities of lemurs parallel the numerical abilities of other non-human primates (such as rhesus macaques or chimpanzees, for example), then it may be possible to infer that numerical cognition in emerged prior to the divergence of lemurs, lorises, and galagos (the prosimians)."

How does this work? Humans and New Caledonian Crows both make tools, but humans are synapsids and crows are diapsids. Are you suggesting that tool making can therefore be inferred in stem amniotes?

I'm not an expert on numerical cognition or evolution, but I think the difference is that in the human/crow case, there would need to have occurred many losses of numerical cognition (in other words, every branch of the amniote cladogram that did NOT lead to either humans or crows would need to have lost the trait). Each of those trait losses makes that explanation less parsimonious, which is why we consider it much more likely that this is a case of convergent evolution (numerical cognition evolved twice, in corvids and in primates). In the human/lemur case, the idea is that N.C. evolved once and is present in all or most of the species descended from that ancestor. Can you think of a reason this doesn't work?

Vaccination, Confirmation Bias, and Knowing Your Audience

jgoldman — Thu, 04 Nov 2010 19:00:34 +0000

Vaccination, Confirmation Bias, and Knowing Your Audience

"When men wish to construct or support a theory, how they torture facts into their service!"

Even in 1852, psychologists like Charles Mackay, who wrote those words in his book Extraordinary Popular Delusions and the Madness of Crowds, were well aware of the dangers of confirmation bias.

I was reminded of the pervasiveness of this cognitive bias last weekend during a visit to GlaxoSmithKline's vaccine distribution facility in Marietta, Pennsylvania. Confirmation bias is as dangerous in 2010 as it was in 1852.

I was invited by David Wescott to join a group of bloggers for an event at GSK's facility during which they would discuss the importance of adult vaccination, and I agreed to participate. My travel, accommodations and food were covered by GlaxoSmithKline. There was no explicit or implicit expectation that I'd write about the trip, and they have not reviewed this post. (Quite an impressive release of control for a multinational pharmaceutical corporation!)

As a science blogger, it wasn't exactly clear why I was included, as science bloggers generally already "get it." We know that vaccination is important, and we often write about it. That public health is a bit beyond the scope of this particular science blog only further served to confuse me. Then I landed in Philadelphia and arrived at the hotel to meet up with David and the other invited bloggers. I found myself surrounded by a relatively diverse group of intelligent bloggers...but, well, they were all mom bloggers, and all relatively local. Why did they fly me all the way out from LA? Why was I there? I still couldn't figure it out.

The next morning, after the two hour ride to Marietta (during which I observed that the changing of the leaves during autumn is NOT a myth!) the program started. It was then that I finally understood why I was there.

I don't write about health, or vaccines, I don't have a family, and I already understand the importance of vaccination. But I do write about the mind. And while all the women were watching the presentations on adult vaccination, I was watching them. At one point, the speakers were discussing the importance of getting inoculated each year against the influenza virus. I don't mean to single anyone out, but one comment was particularly striking: "I've never gotten the flu vaccine, and I've never gotten the flu." As if to suggest that there is no real reason she should have to worry about the flu virus. The very same flu virus that evolves resistance to our vaccinations so rapidly that new vaccine formulas need to be developed each year. The same flu that kills thousands in this country each year. "How they torture facts into their service," indeed.

The thing is, though, that this sentiment, which betrays a certain misunderstanding of epidemiology, statistics, and biology, was not uttered by an anti-vaccination wingnut praying at the altar of Jenny McCarthy. The woman vaccinates her children! She gets it! She is an unfortunate victim - not of anti-vaccination propaganda or the so-called "balanced" journalism - but of the cognitive biases built into her own mind.

Consider a group of people in a coin-flipping tournament. If someone wins a coin flip, they stay in the game. If they lose, they're out. At the end of fifty rounds of coin-flipping, there will be one person left, who will have won the tournament. She will have won the coin-flip fifty times in a row. But there is nothing special about her. When it comes to the fifty-first coin flip, she still has an equal probability of winning or losing. But our minds aren't really equipped to understand this kind of probability.

That brings me to my second observation. For a corporation that depends on communicating science to the public, they did a terrible job of it! For a certified card-carrying data-whore like myself, the Powerpoint presentations (which broke every. single. rule. of effective presentations. I highly recommend that they hire my friend Les Posen to teach them how to present properly) did not have enough detail. They would present some statistic, but without the level of detail required for me to make any real sense of the data. I will grant that I was not the intended audience of the talk, so I will forgive them their lack of error bars and missing p-values. For a general audience without a scientific data-driven background, the presentation was even more useless! It was all statistics, bar graphs, and numbers. If you're going to communicate science to a general audience (and I'd like to think that I know a thing or two about communicating science to a general audience), you need to engage them emotionally. You need to tell a story, not drown them in statistics. The presenter might say something like, "Last year, three gazillion people died because they were not vaccinated against a Terrible Disease That Kills People In Gruesome Ways But Which We Could Eradicate In Less Than A Decade If Everyone Would Just Get Vaccinated." Everyone would agree that this is a Bad Thing, at least. More likely, it is a Terrible, Horrible, No Good, Very Bad Thing.

But the human mind is not capable of really understanding the magnitude of a gazillion. To us, a gazillion and fifty thousand and twenty thousand aren't really so different. For that matter, neither are twelve and sixteen. That is because as the magnitude of a number increases, the specificity of our mental representation of that number decreases. Put another way, as the magnitude of a number increases, the margin of error in forming an accurate mental representation of that number increases as well. This is also true for other animals, like monkeys and rats and fish.

Remember this study?

Giving us incidence and death rates and other such statistics doesn't really get the job done. It doesn't communicate what they want it to. Nor will glossy pamphlets (like the one they gave me) featuring Mia Hamm telling us to get vaccinated. What will get the job done is story-telling, appealing to emotion, and utilizing accessible analogies. Instead of telling us how many gazillions died last year, tell us how many airplanes full of people, or how many football stadiums full of people died last year.

The people who work on developing vaccines put years and years of effort into developing their products. And corporations like GSK spend millions of dollars funding those efforts. It is a shame that they can't communicate the critical importance of vaccination in a more effective way.

As we left the facility, I turned to David and quietly asked him, "do companies like this have any psychologists in their communications departments?" "No," he said, "they're all PR and marketing people."

Perhaps they should consider hiring on a cognitive psychologist or two.

Addendum: It occurs to me that GSK's legal department must carefully scrutinize any sort of public communication, and perhaps the presenters we interacted with were severely restricted in terms of content and delivery style. That said, there must be a way to communicate science in a more robust way that still falls within the confines of legal acceptability. An interesting question to ponder.

Here's the disclaimer again: My travel, accommodations and food were covered by GlaxoSmithKline. There was no explicit or implicit expectation that I'd write about the trip, and they have not reviewed this post.

Learn more about vaccines, courtesy of our friends at Science-Based Medicine.

Posts from some of the other bloggers at the event:
Nutgraf
Mom to the Screaming Masses

Coverage from the official GSK blog: More Than Medicine.

Image source.

jgoldman Thu, 11/04/2010 - 15:00

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decision-making

Parenting and Families

http://lizditz.typepad.com/i_speak_of_dreams/2010/10/the-big-list-of-re…

science communication

I read one of the mommy blogs you linked. Very, very encouraging. Especially the comments section.
But your appraisal of the psychology of effective communication was powerfully demonstrated. The blogger was extremely impressed by the whole process from development to approval. She describes the level of meticulous detail by how she observed an entire lot being discarded because of a labeling problem.
In the comments, the personal stories of dealing with H1N1, babies dying of pertussis, older family members who required iron lungs from contracting polio, etc are definitely made the case for vaccines. Probably more than dry numbers.

Very encouraging to read all this from non-scientists who truly understand the importance of vaccination,

I think all this pushing of flu vaccines is counter-productive: we have people failing to immunise their children against genuinely dangerous diseases and then there's all this flu-related hysteria which seems to be aimed at confirming them in their scepticism through the not-very-credible insistance that they submit to a commercially-driven goldmine of half-arsed vaccination against a virus which carries almost no more than the insignificant risk of having to lose 4 days off work and which will evolve in just a few months into a virus which your recent immunisation will fail to cover anyway.

If I were GSK I would be very excited about the prospect of signing-up 7 billion people to the idea of paying twice a year for an almost completely pointless vaccination.
And if I were an anti-science anti-vaccinator I would regard this as a godsend for me to get my message out there.

Additionally, the PR-driven hysteria surrounding H1N1 resulted in vaccines being rushed onto market without proper testing with the inevitable result that some had to be recalled due to unforeseen side-effects (on children! Tabloids loved that!) and governments wasted bucketloads of tax$$$ over-stocking.
That didn't help at all for flu vaccine credibility.

I used to work in the sales division of GSK. I can confirm your suspicions that that it, and all the other pharmaceutical companies, have a public face (and strategic mission) conceived and executed by their marketing departments, and then constrained by their legal departments.

Since about ten years ago, all marketing materials are rigorously sanitized of any intellectual interest by the legal department, especially the PowerPoint slide presentations - which must be presented as is and in entirety, from start to finish.

Speakers can go off topic or use their own slides only if an attendee asks a specific unsolicited question. It makes for dreadful presentations if audience members have not learned to interrupt with unsolicited questions. ;D

You can't blame pharma, though - look at the huge fines levied on individual companies for promotion off-label, etc. A number of pharma companies have done some unethical things, but it is also true that the FDA harasses them over truly trivial matters, and sometimes for no apparent reason whatsoever except the malice of an individual committee member.

It is a very antagonistic relationship - the notion of denialists that pharma and the FDA are somehow coconspirators is pretty absurd to anyone who has worked in the industry.

"If I were GSK I would be very excited about the prospect of signing-up 7 billion people to the idea of paying twice a year for an almost completely pointless vaccination. "

In its heyday, the relative contribution of vaccine sales to GSK's bottom line was so small as to be not even worth mentioning at sales meetings. Vaccine production is cost-intensive and generally has a very small profit margin.

There are very few pharma companies still making vaccines these days.

Hi Jason,

You may not know that this week was declared "Vaccine Awareness Week" by two vigorous opponents of vaccination, Joe Mercola and Barbara Loe Fisher. Drs. Gorski and Novella encouraged thinking bloggers to, well, oppose their efforts.

As I often do, I am collating the response at my blog:

Incredible discussion on how to communicate science to a general audience. I look forward to more discussion about successful communication.

I also think you may consider doing a post for journalists about how to prevent making gruesome errors in reporting.

Jason, ta for the referenced and link. You'll be interested to know that in my presentation skills workshops for scientists I have a very early section which serves to justify why scientists need to come to terms with how audiences are receiving information nowadays: through TV, movies, YouTube, and the 24/7 news cycle. One clip I show comes from Diane Sawyer's ABC current affairs program where she discusses the American Paediatric Association's review of the literature on Autism and its treatment. Instead of going into detail, she cuts to an interview with Jenny McCarthy who implies scientists to listen to her anecdotal evidence. Sawyer then cuts back and says if you want more information especially from scientists go to the ABC website blog.

That always gets a chuckle from my audiences, because the emphasis in the story is the celebrity over the scientist. But that what presenters in the science are up against in 2010.

The Origins of Small Number Representation

jgoldman — Thu, 19 Aug 2010 08:15:00 +0000

The Origins of Small Number Representation

Earlier this week I wrote about the developmental and evolutionary origins of large number representation. A series of studies in human infants, monkeys, rats, and fish demonstrated that animals and humans spontaneously represent large (>4), abstract, approximate numerosities. Animals, human infants, and human adults, show the same ratio signatures (based on Weber's Law). Adult tamarins are on par with 9-month-old human infants. With age or training, discriminability becomes more precise, and the the critical ratio is reduced a bit. There is good evidence that the large number cognitive system is evolutionarily-ancient and non-verbal, and is innate.

What about for small numbers? Small numbers do not need to be counted or estimated; instead, they are subitized. Upon seeing a scene with a small number of objects you have a sudden, immediate sense of how many objects there are. This happens in parallel rather than serial- you do not need to count the items individually. Therefore, judgments made about displays of 1, 2, 3, or 4 items are rapid, accurate, and confident. As the number of items in the scene increases, judgments are increasingly less accurate and made with less confidence. Response times also increase, with an extra 250-300 milliseconds added for each additional item to be counted beyond four.

Previous work has demonstrated that animals have some pretty sophisticated numerical abilities, but often these experiments depended on a small number of highly trained captive animals, in highly artificial testing situations. What about spontaneous number representation in a population of non-trained free-ranging monkeys? Or human infants?

Cayo Santiago is a small island off the coast of Puerto Rico that is home to approximately 1,000 free-ranging rhesus macaques living socially. The fact that these monkeys are able to thrive in social groups isolated from any natural predators makes them an ideal population to study, and since they are so used to humans on the island, it is relatively easy to approach and work with them.

Figure 1: A rhesus macaque on the beach at Cayo Santiago.

The monkey study had two goals: (1) establish at least one natural context in which number is encoded spontaneously, and (2) establish the limits of this capacity, as a point of comparison for other animal studies, and for studies of human infants.

Two hundred male and female rhesus macaques (Macaca mulatta) living on Cayo were tested. Two researchers placed themselves two meters apart from each other, and 5-10 meters away from the monkey. Each person had a bucket and each bucket was of a different color. The researchers indicated to the monkey that the buckets were empty by tipping them over and placing their hands inside. Then the buckets were placed on the ground in front of the researchers' feet. The first researcher placed one or more apple slices into the bucket, one at a time, making sure the monkey was watching, then stood up, and looked back down at the bucket. Then, the second researcher did the same thing with the second bucket. Both researchers then turned and walked away in separate directions at a natural pace. Since the experimenters no longer appeared to be watching the buckets, the monkeys were able to approach one of them to retrieve a snack.

First, they made sure that the monkeys preferred apples in the first place. One researcher puts a rock into his bucket, and the other puts an apple slice. 15 out of 15 monkeys approached the bucket that contained the apple slice.

After settling that, the researchers went on with the rest of the experiment: how proficient were the subjects at comparing quantities of apple slices? Everything was counterbalanced: sometimes the greater quantity was presented first, and sometimes the lesser quantity was presented first. Sometimes experimenter A presented the greater quantity and sometimes he presented the lesser quantity. Sometimes the greater quantity was on the right side, and sometimes it was on the left side. All quantities were presented sequentially, to prevent the monkey from making a judgment based on a correlate of number, such as volume or surface area.

Figure 2: Results

Condition A was the apple slice (1F) versus the rock (1NF). Conditions B-J are all varying quantities of apple slices. The subjects preferred the larger quantity for comparisons of 1:2 (B), 2:3 (C), 3:4 (D), and 3:5 (G). For comparisons of 4:5 (E), 5:6 (F), 4:6 (H), 4:8 (I), and even 3:8 (J), the monkey did not differentiate the quantities.

Since each monkey was only tested once, it could not have learned anything useful about the task. Instead, it must have spontaneously kept track of the quantities of apple slices in each bucket and represented those quantities in memory. The monkeys also must have been able to compare the relationship of the two numbers in memory by establishing an ordinal relationship, since they always chose the box with the larger number of apple slices.

This experiment suggests that monkeys spontaneously distinguish between sets of 1, 2, and 3 objects. and can further differentiate 3 from 4 or 5 objects.

Recall that the cotton-top tamarins were able to distinguish 8 from 16 items. If there was only one number system that depended on ratio, and success was found for 8:16, then you might expect that 4:8 would yield success as well, in this experiment since it is the same 1:2 ratio. But this was not the case, suggesting that there are two distinct systems for number representation: a large number system, and a small number system for quantities less than or equal to four.

This study was repeated with human infants. Instead of using apple slices, they used graham crackers. Does the small number system in infants depend on a ratio signature, as with the large number system? Or do they represent and encode individual objects in sets of about 4 or fewer items like rhesus macaques?

The experiment included 124 infants, half of whom were 10 months old, and half of whom were 12 months old. Approximately half of the group were males, and half were females. Graham cracker pieces (of identical size) were removed from a small plastic bucket and placed into two opaque containers, which were too tall for the infant to see inside. Each infant sat on the floor, about 1 meter away from the experimenter.

As with the monkey study, the researchers first ensured that the infants were able to approach one of the boxes. The infant watched a toy being placed into a bucket, and was encouraged to crawl to the bucket to retrieve the toy.

Once this had been established, the researcher showed the infant that both containers were empty. She placed them on the floor between herself and the infant, at equal distances from the infant. All crackers were placed sequentially, just as in the monkey study, with similar counterbalancing procedures. After the presentation of the crackers, the experimenter looked down to avoid providing subtle cues to the infant. If the infant did not approach within 10 seconds, the experimenter provided verbal encouragement, but did not look up or make eye-contact with the infant. A choice was recorded if the subject reached into one of the containers, or approached and sat in front of the container for at least 8 seconds without reaching in. Infants who examined both containers were excluded from analysis.

Figure 3: Data.

Infants in both age groups chose the container containing the larger number of graham crackers in the 1:2 condition and the 2:3 condition. For 3:4, neither age group showed a preference for the greater number, and they were also at chance for 2:4 and 3:6. These results indicate that infants recognize more/less relationships. Again, since each infant participated it only one trial, there was no chance to learn the task. They had to spontaneously track the graham crackers, establish the numerical relationship for both quantities, and compare them.

The set-size effect for the infants in this study was three: they discriminated 1:2, and 2:3, but not 2:4 or 3:4, or even 3:6. The monkeys were able to discriminate 3:4 and 3:5, though they were adult monkeys, and the sharpened ability to discriminate small numbers may have grown through experience.

Given the monkey and infant studies, there is reasonable evidence to carefully conclude that the small number system is probably innate, especially considering the common signatures.

But to really drive the point home, we should consider one more study, of infant chickens, in just the first few days of their lives. Chickens turn out to be a really good model organism for study in the laboratory, since they are precocial. Chickens do not need any parental care - that is, starting from the day they hatch, they can feed themselves, find water, and generally survive. Further, upon hatching, their motor systems are such that they can walk around and interact with the world, and their visual systems are well developed. Compare this to human infants who are severely dependent on parental care, can't walk around until one year of life, and don't have decent visual acuity for several months. By the time human infants can participate in a reaching or looking preference experiment, they've had at least several months of experience in the world. For an investigator interested in innate processes separate from experience, chickens are a perfect species. Further, with a cleverly designed lab set-up, the chicks' experiences can be carefully controlled.

Rosa Rugani and colleagues, from the University of Trento in Italy, used chickens to test whether or not the small number system previously seen in infants and monkeys could be found in chickens. The chicks were imprinted to a small ball. This is critical because when chicks are separated from their imprinted object, they have a strong drive to reunite with it. Moreover, it is known that given a choice between hanging out with one of their imprinted objects (one ball), and several imprinted objects (several balls), they will always choose to hang out with the larger number of imprinted objects, when all items are visible. When it came time for testing on the third day of life, the chicks were placed in a small holding chamber with a transparent door while they watched balls that were identical to their imprinted object disappear behind one of two opaque walls. Then they were released from the chamber and allowed to approach one of the walls.

Figure 4: Testing apparatus. The holding chamber can be seen on the left, and the two opaque walls on the right.

Each chick participated in two sessions of 20 trials each. Success on a trial required a simple comparison between 2 objects and 3 objects. In one condition, the balls disappeared behind the walls simultaneously, and in a second condition, they were placed behind the walls consecutively. The chicks chose to approach the screen hiding three balls more often than expected by chance in both the consecutive condition (CDT) as well as the simultaneous condition (SDT). There was no difference in success rate between the two conditions.

Could these results have been the product of learning? In an additional analysis, only the first five trials from each session was analyzed, and the statistical significance of the results was preserved. These results confirm that infant chicks spontaneously discriminated sets of 2 and 3 objects, preferring the larger set, even when they must do so on the basis of short-term memory.

Figure 5: Results.

However, it is possible that the chicks were relying on a correlate of number instead of the actual number of items, in making their decision, such as surface area or contour length. A second experiment was therefore conducted with new chicks, that was identical to the first, except for one key change. Instead of balls, the chicks were imprinted to and tested with red squares. The experimenters were able to systematically control the properties of squares so that the set of three squares would have equivalent total surface area or contour length as the set of two. As before, each chick participated in 20 trials on day 3 of life. The results of this experiment confirmed the prediction that the chick was using the small number system, and not relying on some correlate of number: chicks preferred to approach the screen hiding the larger set in all conditions.

These studies, combined with the ones discussed on earlier this week, suggest that:

There is one system that approximates sets of large numbers, and distinguishes among two sets of large sets on the basis of a ratio determined by Weber's law.
There is a second system for determining the exact amount of object in a set of small numbers, when the number of objects is four* or less.
There is good evidence that each of these systems is innate, though they can be sharpened with age or experience.

The importance of these studies may not be immediately obvious or apparent. But I think these experiments are important in figuring out how the human mind evolved, which cognitive capacities are innate and evolutionarily-ancient, and which require some amount of learning or experience to emerge.

*Shouldn't this number be the magical 7? Early research determined that short-term memory has a capacity for 7 plus or minus 2 items. However, later research suggests that the true magic number is indeed four. That people can remember ~7 items is instead due to "chunking" of larger sets of items four or fewer smaller sets.

Hauser, M., Carey, S., & Hauser, L. (2000). Spontaneous number representation in semi-free-ranging rhesus monkeys Proceedings: Biological Sciences, 267 (1445), 829-833. DOI: 10.1098/rspb.2000.1078

Feigenson L, Carey S, & Hauser M (2002). The representations underlying infants' choice of more: object files versus analog magnitudes. Psychological science : a journal of the American Psychological Society / APS, 13 (2), 150-6. PMID: 11933999

Rugani R, Fontanari L, Simoni E, Regolin L, & Vallortigara G (2009). Arithmetic in newborn chicks. Proceedings. Biological sciences / The Royal Society, 276 (1666), 2451-60 PMID: 19364746

jgoldman Thu, 08/19/2010 - 04:15

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birds

chicken

Comparative Psychology

mammals

Rhesus Macaque

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Categories

Life Sciences

Love this one!

Very nice post.

Haven't looked at this subject since reading about corvines and shotguns!

What Are The Origins of (Large) Number Representation?

jgoldman — Tue, 17 Aug 2010 07:30:00 +0000

What Are The Origins of (Large) Number Representation?

This post considering the evolutionary origins of numerical cognition, specifically in terms of the approximation of large numbers, is meant as a companion to this week's series on the developmental origins of numerical cognition and developmental dyscalculia, at Child's Play.

What are the origins of number representation in the mind? Are there any innate building blocks that contribute to our understanding of mathematics and number, or must everything be learned?

Number is an important domain of human knowledge. Many decisions in life are based on quantitative evidence, sometimes with life or death consequences.

Figure 1: Fight or flight?

By now you probably have come to expect that I'll be arguing that there are several innate "building blocks" of cognition that give rise to more complex mathematics. To start with, what are some of the arguments proposed by the empiricists?
(1) Number knowledge is entirely conceptual - it requires seeing objects as belonging to sets;
(2) Number knowledge is abstract. You need to understand the similarity between 3 people, 3 objects, 3 sounds, 3 smells, 3 dollars, 3 seconds, 3 hours, and 3 years;
(3) It doesn't appear to be cross-culturally universal. Some cultures have more advanced mathematics than others; and
(4) babies and monkeys can't do long division.

Surely, humans have something unique that allows us to do things like multivariate regression and construct geometric proofs, however, but let's start at the beginning. I will hopefully convince you that there is an evolutionarily-ancient non-verbal representational system that computes the number of individuals in a set. That knowledge system is available to human adults and infants (even in cultures that don't have a count list), as well as to monkeys, rats, pigeons, and so forth.

In the first study (of human adults) that we'll consider, participants were presented with arrays of dots on a computer screen that were presented only for a fraction of a second. In that time, the participants had to determine if the second array had more or fewer dots than the first array. They controlled or counterbalanced dot size, density, shape, and things like that. How did they do in this task?

Figure 2: Results. They were at chance on trials comparing 32 and 34 dots (center). All other comparisons, adults demonstrated above-chance discrimination.

Here's the important point. If they were counting, then they should have been able to discriminate 32 versus 34 dots just as easily as they were able to discriminate 8 versus 10 dots. Also, if they were counting, it should have taken then longer to count 32 versus 64 dots, than for 8 versus 16 dots. Since the dot arrays were displayed for the same amount of time, and they were able to discriminate both of those sets of numerosities equally well (and perfectly), it follows that they weren't counting.

It turns out that the discriminability of two numerosities depends not on the total number of objects, but on the ratio between the two numerosities. Notice that 32 vs. 64 and 8 vs. 16 have the same 1:2 ratio. This also implies that the mental representations of these numerosities are inexact - they are approximations, not exact numbers.

Figure 3: Where the ratio was 1:2, responses were perfect. Success rate decreased as the ratio decreased. When the ratio was 1:1.1, success was basically at chance, but considerable success at 1:1.15.

Is this limited to the visual domain? In the next experiment, participants saw a similar array of dots, and then they heard a set of sounds. This cross-modal comparison (73% accuracy) was almost as accurate as the visual comparison alone (76% accuracy). The participants were also asked to add sets of objects. They were shown two arrays sequentially, and asked to add mentally approximate the total number of dots on both arrays, and shown a third array. They were asked if the sum of the dots in the first two arrays more or less than the total amount of dots in the third array. This resulted in 72% correct responses. Last, this was done cross-modally. They were shown a dot array, then given a sequence of sounds, and asked to add them. Was the sum of the dots and the sounds more or less than the total amount of dots in a new, third, array? Accuracy was 74%. Accuracy was roughly equal across all four of these conditions, suggesting that numerical representations are abstract. It also means that these approximate abstract representations contribute to our ability to add.

Figure 4: Results. Equal success for each condition.

"But," the empiricist says, "these representations have been mapped onto verbal numerals. Even though you might have an approximate estimation of the number of dots, you still use language. You might think 'that's about 50 dots' or 'it seems like there's around 300 dots.' These people have spent years learning and using formal arithmetic." And in response to the empiricist, the nativist says: "Fine. Have it your way. Bring on the babies and animals."

So we gather a group of six month old babies who don't have language yet. We can't ask babies which array has more dots, so we use a habituation paradigm. We show the babies arrays with the same number of dots (say, 8 dots) until they get bored of it and spend less time looking at it. Then we show them a new array (say, 16 dots). Do they look longer at the new array of 16 dots than at a new array of 10 dots? If so, that means that they discriminate 8 versus 16, but not 8 versus 10.

And so it was. Infants successfully discriminated 8 vs. 16, 16 vs. 32, and 4 vs 8. They failed to discriminate 8 vs. 12, 16 vs. 24, or 4 vs. 6 dots. So infants, too, show a ratio limit, though the critical ratio is higher for infants (1:2) than for adults (1:1.15).

How general is this ability? Does it work for sounds as well? A new group of six-month-old and nine-month-old infants were placed between two speakers, and were habituated to a number of sounds coming from a certain direction. Instead of looking time, the measurement was whether or not the infant turned his or her head to orient toward the source of the sounds. They were familiarized to, for example, 8 sounds or 16 sounds, and then tested with 8 and 16 sounds. The findings from this task were similar to the findings in the dot task.

Six-month-olds discriminated 8 vs. 16 and 4 vs. 8 sounds, but failed at 8 vs. 12 and 4 vs. 6. The nine-month-olds discriminated 8 vs. 12 and 4 vs. 6 sounds, but failed at 8 vs. 10 and 4 vs. 5 sounds. Again, discrimination of numerosities showed a ratio limit, though discrimination gets sharper with age.

Let's push the question a bit farther. Approximate numerical representation exists for visual objects as well as auditory sounds. What about for actions?

Infants were habituated to a cartoon in which a rabbit jumped either 4 times or 8 times. The total distance of the movement of the rabbit was equivalent, such that the each of the 4 jumps were twice the distance of each of the 8 jumps. This allowed the rabbit to end up at the same location at the end of each set of jumps - so the infants couldn't rely on physical displacement as a correlate of number of jumps. The findings were the same as the dots and sounds. Representation of number is truly abstract, even in infants.

To recap: Before learning to count or learning arithmetic, infants represent and discriminate large numerosities. These representations are approximate, and subject to a ratio limit. These representations are also abstract, and the same ratio limits applies to objects, sounds, and actions. This capability is present in infancy, though it increases in precision through development.

On to the animals.

In the 1950s and 1960s, Dr. Francis Mechner did a series of conditioning experiments with rats. In one such study, rats were trained to press a lever 4, 8, 12, or 16 times in order to receive a reward. Tension on the lever was controlled, so that the rats couldn't rely on total effort as a correlate of the number of level presses.

Figure 5: Presses on the lever by rats.

These data indicate that rats also have inexact, approximate representations of large numerosities, and that there is also a ratio limit. Accuracy decreases as the target number of lever presses increases.

Some years ago, a group from Harvard investigated this question in cotton-top tamarin monkeys as well. They did the same auditory discrimination experiment with the monkeys as had been done previously with infants. The monkeys were habituated to a sequence of sounds coming from the right or left, and then presented with a new number of sounds. Again, the turn of the head to orient towards the sound was used as an indication of discrimination. Their performance was similar to the 9-month-old infants: they discriminated 4 vs. 6 and 8 vs. 12, but not 4 vs. 5 or 8 vs. 10. They could discriminate 2:3 ratios, but not 4:5 ratios.

How evolutionarily-ancient is this cognitive capacity? Our common ancestor with tamarins is relatively recent, compared to say, with fish. So let's look some Italian fish.

Figure 6: Eastern mosquitofish (Gambusia holbrooki). This one is about 4cm long.

Female mosquitofish like to hang out with groups of other females as protection from sexually harassing males.

So you take a female mosquitofish, and you let her habituate to the fishtank. On either side of a long fishtank there are two additional groups of females of differing sizes, in their own tanks.

Figure 7: Something like this.

Meanwhile, you take a male mosquitofish and you deprive him of any females for a whole week. Sucks to be him. You introduce him into the tank with the female, and its game on. He desperately wants the feel of her cold, wet, slimey, scaley body. He can't wait to make sweet fishy love. He sees her from across the tank. He works up his nerve and says, (channeling his best Walter Matthau impression) "Maria, there may be lots of fish in the sea, but you're the only one I want to mount over my fireplace."

If he makes at least 10 attempts to have his way with the female in the first five minutes, then you record which group of fish the female tries to join. She should prefer to hang out with the larger group of females.

When the two groups of female fish were different according a 1:2 ratio, she always chose the larger group, but when the ratio was 2:3, she chose randomly. Just like the monkeys, and just like the human infants.

Figure 8: Success for 1:2 ratios, but not for 2:3 ratios.

What can we conclude from this series of studies?

Animals and humans spontaneously represent large (greater than 4), abstract, approximate numerosities. Animals, human infants, and human adults, show the same ratio signatures. Adult tamarins are on par with 9-month-old human infants. With age or training, discrimination ability becomes more precise, and the the critical ratio is reduced a bit.

The large number cognitive system is evolutionarily-ancient and non-verbal, and is likely innate.

Next up, later this week: small numbers.

Barth H, Kanwisher N, & Spelke E (2003). The construction of large number representations in adults. Cognition, 86 (3), 201-21. PMID: 12485738

Lipton JS, & Spelke ES (2003). Origins of number sense. Large-number discrimination in human infants. Psychological science : a journal of the American Psychological Society / APS, 14 (5), 396-401. PMID: 12930467

Mechner F (1958). Probability Relations within Response Sequences under Ratio Reinforcement. Journal of the experimental analysis of behavior, 1 (2), 109-21. PMID: 16811206

Hauser, M., Tsao, F., Garcia, P., & Spelke, E. (2003). Evolutionary foundations of number: spontaneous representation of numerical magnitudes by cotton-top tamarins. Proceedings of the Royal Society B: Biological Sciences, 270 (1523), 1441-1446. DOI: 10.1098/rspb.2003.2414

Agrillo, C., Dadda, M., & Bisazza, A. (2006). Quantity discrimination in female mosquitofish. Animal Cognition, 10 (1), 63-70. DOI: 10.1007/s10071-006-0036-5

jgoldman Tue, 08/17/2010 - 03:30

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But... but... but... uh... the PirahÃ£!!!

(kidding)

Here's one for you. I am a writer with a problem; my brain wants to quantify punctuation, specifically colons, semicolons, and commas. I will begin editing a page using standard rules of grammar, but soon I am using a different system based on a ratio, roughly 1 colon: 3 semicolons: 6 commas. This is not the limit per page, but a ratio for each page. Periods are exempt.

This 'automatic' quantifying of punctuation is a real pain! I have to go over a ms. many times, and while trying to correct the mistakes I am apt to lapse into the ratio system without knowing it.

Excellent post. Thanks!

This powers of two recognition and ability to distinguish doubled up/down numbers brings to mind so called "Peasant multiplication" which was a very common system for multiplication and division which relied only on the ability to multiply and divide by two, and addition and subtraction.

It's also impossible to avoid mentioning binary arithmetic I suppose.

I once got bitten by a mosquito fish; ended up with filaria....

It would seem essential that being able to identify relative numbers and numbers in a group is very basic for animals: a pride of lions must decide whether or not to contest another pride's territory, or discretely move on - do they outnumber us, how many males vs. females and cubs; weight, fitness, health. Even an "inventory" of available prey. What we call mathmatics is embedded in every form and function in nature.

Excellent post.

Is the cotton tamarin experiment the one done by Hauser that is under investigation at this time?
This obviously doesn't take anything from the conclusion or from the other studies, but I am not sure if it's one of the studies that were going to be under evaluation for possible "retraction"

Nope, it is not.

I don't understand two issues here:

1) Almost all the experiments deal with discrimination, so the best case scenario is that they tell us something about the number discrimination process, not about number representation - to this extent - discrimination is sensitive to ratios not pure subtractions - that's the only inference you could make.

2) In the rat study, the rats aren't comparing values (at least as presented here), they are responding to absolute values, and it looks like they do reasonably well. What point is there in saying that they are storing ratios not absolute values? I see the rat study to be showing something completely different from the rest of the studies - it is possibly the only one that could say something about the representation of numbers (as opposed to discrimination).

@10: The idea is in order to discriminate two quantities, you have to mentally represent the magnitude of each quantity. The conclusion made is that discriminations are made on the basis of approximate ratio instead of exact numerosity. The rat study demonstrates, with a different sort of task, that they too represent large numbers approxmiately, not exactly.

yes, discriminations are made with approximate ratios, this does not mean the numbers are stored as ratios.

The rats study was also claimed to have "a ratio limit", I'm not sure I follow this, given that it was about tracking absolute values.

I am not against saying that large number representations are "approximate". It seems reasonable. It is the further claim that somehow they are "ratios" that seems weird - cos the ratios are relevant ONLY for discriminatory purposes where there is a second number for ratios. Otherwise, you couldn't store a number as a ratio.

I forgot to mention: I really like your reviews / posts, so keep 'em coming! cheers!

I'm going to jump in with another anecdote about my "odd" relationship with mathematics. First, I need to mention that I score high on verbal / visual intelligence and have very low mathematical aptitude. As a geology student I faced 2 semesters of calculus based physics and 3 semesters of calculus. Baby stuff, I know, but daunting for me. I decided to take the physics semesters BEFORE the calculus course; it sounds crazy, but it helped.

What is odd is that I figured out how to navigate these "foreign" languages using a visual "code" that I made up, a metalanguage, I guess. I connected a description of problem type (thingy-symbol = thingy) with a check list of how to solve it. Gee whiz, it worked. I had no idea what equations "meant" -

I mention this because I suspect there are other people out there, for whom mathematics is not their "native language" who have had to do something similar.

Zombies Ate My Brain! (and other tales)

jgoldman — Thu, 01 Jul 2010 05:30:00 +0000

Zombies Ate My Brain! (and other tales)

Imagine with me, for a moment, that the zombie invasion has begun. You try to escape, but the zombies are just too much to handle. You can't run fast enough. They're everywhere. Your favorite science bloggers have been turned into zombies and they're coming for you.

Figure 1: Thanks to Joseph Hewitt of Ataraxia Theatre for providing us with these awesome illustrations of zombified sciblings! Left to right: Christie, Sci, Bora, me, & Peter and Travis. Click on each to embiggen.

I'm sure you've always wondered what would happen as a zombie ate through your brain. How would it feel? What would the experience be like? Well, dear reader, I'm here to take you on a tour through the brain buffet. It'll be a smorgasbord of white and dark meat (ha!). A cognitive feast. We will dry rub your dendrites and smoke your axons. It shall be an epic meal of neuronal NOMs. Bacon never tasted so good.

Figure 2: Put on your bibs. This is going to get messy.

First, we need the proper ambiance. One can't properly crunch on cortex or snack on synapses without setting the right mood. Red and white diner tablecloths just won't do. We need white linen, candles, the finest china, expertly polished silverware, and an impeccably dressed waiter who will prepare our meal tableside, as one might a caesar salad or bananas foster, in the finest of fine dining restaurants.

You see, the brain, itself, feels no pain, Clarisse. If that concerns you.

We begin, as Dr. Lecter does, with the dorsolateral prefrontal cortex, or DLPFC. The prefrontal cortex, located at the very front of the brain, is involved in a variety of executive functions. More specifically, the DLPFC is known to be involved in working memory, planning, and decision-making. You would cease being able to speculate about the future; you would be living in the here-and-now. Your short-term memory would be destroyed. The good news, of course, is that without any working memory, you would not be able to transfer the memory of having your brain eaten to long-term memory. You would not be likely to remember this event at all.

Figure 3: Parts of the prefrontal cortex.

The zombies would continue on to the inferior frontal gyrus, which is where you can find Broca's area (on the left hemisphere). Broca's area subserves several functions, but one of the most important is language production. Your language comprehension - the ability to understand others - would be preserved, but your ability to communicate clearly would be lost. You would show the classic symptoms of Broca's aphasia, or expressive aphasia. Speech would be difficult to initiate, and dysfluent. Intonations and patterns of stress, which are important for conveying the emotional content of language, would be lost as well. Most people who are afflicted with Broca's aphasia, after recovery, report that they know what they want to say, but have trouble expressing their thoughts properly. Here is a patient trying to explain how he came to the hospital for dental surgery: "Yes... ah... Monday... er... Dad and Peter H... (his own name), and Dad.... er... hospital... and ah... Wednesday... Wednesday, nine o'clock... and oh... Thursday... ten o'clock, ah doctors... two... an' doctors... and er... teeth... yah." Notice how there is still some meaning to be derived from this, despite the overall structure of the language being lost.

Figure 4: The inferior frontal gyrus, or Broca's area.

Having completed most of the lateral frontal lobe, this has cleared the way to get farther in, towards the insides of the hemispheres, to the medial surface. The zombies would move on to your ventromedial prefrontal cortex, or VMPFC, also occasionally referred to as the orbitofrontal cortex. As the zombies munch through this area, any semblance of appropriate social behavior would be lost. You would not be able to inhibit your more primal, evolutionarily-ancient emotional responses. Your personality would be forever changed. This area is one of the parts of the brain destroyed in the Phineas Gage accident. In the aftermath of that accident, his doctors remarked that "Gage was no longer Gage." Likewise, you would no longer retain the personality that makes you, you.

With most of the anterior section of your frontal lobe gone, what's left but to eat up the very back of the frontal lobe? I refer, of course, to the motor cortex, which can be found on the precentral gyrus. As you might expect, the motor cortex controls the more than six hundred muscles involved in voluntary movements, and can be found at the rear of the frontal lobe, just in front of the central sulcus.

Figure 5: The motor cortex, on the precentral gyrus, and the somatosensory cortex, on the postcentral gyrus.

As we reviewed yesterday, the motor cortex is highly organized, with each area corresponding to a specific set of muscles in the body. The zombies would likely begin on the lateral side, and eat their way up towards the top of the brain, and then down the medial surface. They might begin with the left hemisphere, which would cause paralysis to the right side of your body. You would begin my losing the ability to swallow, chew, or salivate, just before you lose voluntary control over your tongue, jaw, lips, and eventually your face muscles. Next would come your eyebrows, eyelids, and eyeballs. Now you are forced to do nothing but stare straight ahead. The next muscles to go are the ones controlling the neck, before moving on to the limbs: first your thumbs, them the rest of your fingers, the hands, wrists, elbows, and shoulders. Now the zombies eat their way down the medial surface of the motor strip, and you lose the ability to control your torso, hips, knees, ankles, and toes. Moving onto the right hemisphere, the same pattern would occur for the left side of your body. Congratulations! You are now entirely paralyzed.

Figure 6: The layout of the motor cortex.

What's next? Well, the postcentral gyrus of course, on which you can find the somatosensory cortex. Like the motor cortex, the somatosensory cortex is also highly organized. Assuming the zombies follow a similar pattern, you'd now lose sensory feeling in all of those parts of your body, starting with the mouth, moving onto the face, hands, arms, trunk, legs, and feet. You are well on your way to becoming a zombie yourself. Give yourself a pat on the back! Oh wait. You can't. I'd do it, but you wouldn't feel it anyway. Now that you've lost the ability to feel pain, even the most empathetic zombie can continue feasting without worrying for your comfort.

If I was a zombie, the next morsel of delectable delicousness I'd go for would be the superior temporal gyrus, on which Wernicke's area can be found in the left hemisphere. (That's the tastiest part!) Destruction of this area results in Wernicke's aphasia or receptive aphasia, which, contrary to Broca's, impairs language comprehension. Grammar, syntax, intonation are all preserved but the language is meaningless. Patients with this condition generate what is sometimes called "word salad" - intelligible words that appear to be strung together randomly - such as "colorless green ideas sleep furiously." Slightly anterior to Wernicke's area, still on the superior temporal gyrus, is the primary auditory cortex.

Figure 7: Wernicke's area and primary auditory cortex along the superior temporal gyrus. Also shown are Broca's area, and the motor cortex.

At this point, the zombies would probably continue finishing up the temporal lobes. You would certainly lose the rest of your hearing, effectively becoming deaf. Also, since the temporal lobes and occipital lobes are so close, you'd lose all the cortex that binds auditory and visual features together. For example, as the zombies eat their way through the fusiform gyrus (OM NOM NOM!!) of the left hemisphere, you'd lose the ability to bind letter sounds to their visual forms. After eating the right fusiform gyrus, you'd lose the ability to recognize individual faces and become prosopagnosic. Object recognition would still largely be intact - lucky you!

Figure 8: It is surprisingly difficult to find a good image of the fusiform gyrus. There it is, area 37, towards the bottom of the temporal lobe.

The zombies must be getting full by now, having dined on your frontal and temporal lobes, and even part of your parietal lobes already. But the job of a zombie is never over! They must continue to eat! But first, a musical number.

This dude cracks me up.

Alright, let's get back to the brains. The next tasty snack will be the remainder of your parietal lobes. In one big bite, you'll lose your mental number line, spatial orientation, and navigation. (mmmmm...tastes like chicken!) You'll lose your ability to engage in symbolic thought: the ability to represent reality using abstract concepts.

Figure 9: Nice little animation showing the parietal lobe.

All that's left now, of course, is the occipital lobe. The occipital lobe is almost entirely devoted to vision. With the right occipital lobe, you'll lose your left visual field, and with your left occipital lobe, you'll lose your right visual field. Now you'll be blind.

Is there anything left? Of course there is. There are the subcortical structures - hippocampus, thalamus, amygdala, and so on - and the brainstem and cerebellum. But the zombies are pretty full by now. They'll leave some other zombies to get the the subcortical scraps. Maybe they'll feed the scraps to The Dog Zombie.

Want to explore the brain a little more? Many of the images above come from the Sylvius Neuroanatomical Reference, which I love. It's pretty cool, and they even have an iPhone app!

[UPDATE: PZ is keeping a running list of Scienceblogs zombie sightings here]

jgoldman Thu, 07/01/2010 - 01:30

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you tricked me -- I learned something.

A Bonobo in the Hand or Two Chimps in the Bush?

jgoldman — Fri, 11 Jun 2010 05:30:00 +0000

A Bonobo in the Hand or Two Chimps in the Bush?

Bonobo Week continues! I'm donating whatever proceeds I receive from my blogging shenanigans for the entire month of June to help the bonobos at Lola Ya Bonobo.

Imagine that you're wandering in the desert and you come across two magic lamps. One lamp grants three wishes. It's your standard sort of magic lamp with a genie in it. (No wishing for extra wishes, of course.) The second magic lamp is, well, a moody magic lamp. It's inconsistent. Sometimes it grants one wish, and sometimes it grants seven wishes. But the thing is, you don't know for sure whether, when you rub the lamp and genie pops out, if he's going to grant you just one or the full seven. But let's make things more interesting. You only get to use one of the lamps. As soon as you rub one of the lamps and the genie comes out, the other lamp disappears. And you are in the Desert of Infrequent Lamps. Tomorrow you could chance upon two more lamps, with the same rules. But you might not come across any more lamps for many days. So which lamp will you decide to use?

Figure 1: If you're lucky, the genie will have the voice of Robin Williams and will sing to you.

Decades of studies indicated that, as humans, we tend to avoid risk. When it comes to potential gains, we prefer the safe option over the risky option. But in the Desert of Infrequent Lamps, you might be tempted to take the gamble, especially since you don't know when you'll be lucky enough to stumble upon your next lamp. Resources (in the shape of magical wish-granting lamps) are scarce. After all, you're wandering through the Desert of Infrequent Lamps. You gotta get what you can, when you can. Sucks to be you.

Animals face similar risks on a daily basis, though in the context of things like food acquisition and predator avoidance. So it makes sense that natural selection would, over generations, favor certain cognitive decision-making mechanisms that most effectively addressed those risks. Risk preference patterns in animals are variable though. That variability in risk preference has been observed, at least under experimental conditions, suggests that animals can adjust their strategies given the parameters of the immediate environment. For example, when the riskier option may not be very costly, or when plenty of food is available in the environment, the animal may opt for the riskier choice, and under those circumstances that may actually be the optimal decision.

But is there a relationship between foraging ecology and cognitive decision-making mechanisms? Or are observed inter-species differences in risk preference simply due to experimental task demands? This is what a team of researchers from Harvard and Duke (including our hero this week, Brian Hare) wanted to figure out. They hypothesized that differences in feeding ecology in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) have resulted in differential patterns of risk preferences.

Chimpanzees and bonobos are phylogenetically closely related; they diverged from their common ancestor less than one million years ago. They are morphologically and behaviorally very similar, though striking differences have been found in terms of dominance structure, sexual behavior, and aggression. Most important for this study, the two species live in geographically different areas, with different resources. Several previous studies observed that while chimps and bonobos rely mainly on fruits and terrestrial vegetation, bonobos may rely more heavily on vegetation which is found in greater abundance both in terms of space (i.e. amount) and also in terms of time (i.e. available throughout the year). By relying more heavily on their vegetable crudite, bonobos may avoid some of the risk that chimpanzees must contend with in their "frugivorous foraging." (New goal: use the word "frugivorous" in a journal article.) It is also possible that there are more or larger fruit-bearing tree patches available to bonobos, resulting in less competition for that fruit, when compared with chimpanzees. Another key difference is that chimpanzees hunt monkeys for their meat, which is both costly and risky, though has a high pay-off. Taken together, it appears that bonobos have more reliable food sources, and chimpanzees have less reliable food sources, which may result in increased competition. This leads to the hypothesis that, compared with bonobos, chimpanzees would be more risk prone. And compared with chimpanzees, bonobos would be more risk averse. Put in other terms, bonobos live in the Forest of Many Lamps. Why try for the riskier 1-wish or 7-wish lamps when the 3-wish lamp is a sure thing? Especially since there are plenty more lamps to rub. Chimpanzees, however, live in the Forest of Infrequent Lamps. Why choose the 3-wish lamp and risk missing out on four potential additional wishes, especially considering that you don't know when you might find another magical wish-granting lamp?

Figure 2: Researcher Brian Hare with Malou, a bonobo from Lola. Click to embiggen.

Five chimpanzees (3 males, 2 females) and five bonboos (3 males, 2 females) were tested at the Primate Research Center at the Leipzig Zoo, in Germany. All ten apes were born in captivity, were never food deprived, were socially housed, and had ad libitum access to water, including during testing. All had previously participated in other experiments of cognition and behavior. Both species were fed fruits and vegetables every day, and cooked meat once per week (this pattern was maintained throughout the testing period). Since the animals were born in captivity and had equal easy, regular, predictable access to food, any differences in risk preference behavior when it came to food can reasonably be ascribed to evolutionary differences.

First, the researchers assessed number discrimination: for example, could the apes discriminate four grape halves from seven? All participants had sufficient number discrimination, and therefore could participate in the experiment.

Figure 3: Experimental Apparatus. Chimps and bonobos chose between fixed and risky rewards, hidden under the bowls.

In the forced-choice task, the individual would be presented with two upside-down bowls that differed both in color and shape. Under one bowl, four grape-halves could always be found. Under the second bowl, either one grape piece or seven grape pieces could always be found, with equal probability. This meant that in half the trials, the second bowl covered up one piece, and in half the trials, the second bowl covered seven pieces. The four pieces bowl represented the fixed reward, while the one-or-seven pieces bowl represented the risky reward.

Before the test began, the individual was familiarized with the reward contingencies associated with either bowl. Sometimes just the four-pieces bowl was presented, and sometimes just the one-or-seven pieces bowl was presented. After completing both the number-discrimination and the familiarization trials, the individual completed three testing sessions, each with twenty forced-choice trials.

Figure 4: Results. Bonobos in slashed bars, chimpanzees in black bars. Values represent the proportion of trials when the fixed option was chosen, with standard error.

The results are pretty straightforward. Chimpanzees were risk-seeking, significantly preferring the one-or-seven risky option, both within each session and combined across the entire experiment. In fact, chimpanzees became slightly more risk-seeking as the sessions progressed. Bonobos, in contrast, were risk-averse, significantly preferring the the reliable four-pieces bowl. Comparing the two species, chimpanzees were significantly more risk-seeking than bonobos. Comparing individual response patterns, four out of the five chimps displayed risk proneness, and all five bonobos displayed risk aversion. There was no effect of sex or age.

The difference could not have been due to numerical cognition, since both species were highly successful at choosing the larger reward in the number discrimination trials. That they could not only discriminate, but consistently chose the larger amount, suggests that the differences were not due to motivation. Chimpanzees chose successfully 95% of the time on these trials; bonobos 94% of the time. Both species were sufficiently motivated to acquire the larger rewards.

The chimps and bonobos tested in this experiment used highly different decision-making strategies when confronted with the same task. Chimpanzees preferred risky choices, while bonobos played it safe. Because there were slight differences in living conditions in the zoo as well as general cultural differences between the two species, the effects of experience can't be completely ruled out. However, the fact that these animals were all born in captivity and had similar and reliable access to food and water suggests that these differences reflect decision-making mechanisms that emerged due to natural selection because of differential environmental circumstances, and was not due to experience.

Additionally, converging evidence for these findings comes from a different study concerning decisions regarding delayed rewards. Chimpanzees were willing to wait significantly longer for larger rewards, compared with bonobos. In this case, the increased delay could represent a significant risk. Chimps may be more willing to wait longer for a larger reward because they are more willing to incur the risk.

At this point, you may be asking why we should care about the risk preference patterns and decision-making mechanisms of bonobos and chimpanzees. And one answer is that humans didn't evolve to engage with modern economics. Indeed,

many of our preferences are probably tailored to providing adaptive foraging and other evolutionarily relevant decisions. An evolutionary approach to economic preferences can therefore offer keen insights into the nature of human and animal decision-making.

Heilbronner, S., Rosati, A., Stevens, J., Hare, B., & Hauser, M. (2008). A fruit in the hand or two in the bush? Divergent risk preferences in chimpanzees and bonobos Biology Letters, 4 (3), 246-249 DOI: 10.1098/rsbl.2008.0081

jgoldman Fri, 06/11/2010 - 01:30

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