Once again, I bravely plunge into the fascinating world of kinky sex research in humans. This time, we learn something incredibly useful. Gentleman, would you like to know how to improve the potency of your semen? Do you need a good excuse to give your significant other when she catches you browsing porn sites? Do you want another excuse to sneer at those pompous business types who flaunt their fancy cell phones? Here's the study for you.
There's a vested interest in bettering male sperm quality at fertility clinics, obviously. There are multiple ways to measure the potency of a semen sample: one is to look at sperm density, which is typically in the neighborhood of 70 million sperm per millileter. Another is to look at sperm quality, measured as the percentage of functional, motile sperm in a sample. These are routine assessments that the professionals at fertility clinics make on a regular basis, and are easily quantified.
Now for a little theory: if males can regulate their sperm quality (as we'll see, we apparently can), then the situations in which we ought to be putting a little extra oomph into the boys is when there is a possibility of competition: if we're concerned that someone else might be introducing sperm into our favorite vagina, we ought to make a special effort to put our best wigglers in the race.
How can this be tested? With the judicious use of pornography. A group of young heterosexual men were asked to masturbate into a cup, while provided with a randomly selected set of graphic images for stimulation. The set to elicit feelings of sexual competition consisted of explicit images of two men with a woman; the less competitive images were of three women. The fellows filled out a little questionnaire that asked miscellaneous details, were handed a cup and some calipers (they also had to measure the size of their testes) and a sealed packet with their porn, and returned a little later with their output.
The results fit the hypothesis. Males who viewed the hardcore shots of other men with a woman produced more sperm (76.64±1.26 x 106/ml vs. 61.35±1.27 x 106/ml), and a higher percentage (52.1±7.3% vs. 49.3±8.0%) were actively motile. The men who thought the photos were more explicit than any they had seen before also had more potent semen than those who thought the images were relatively tame—the effect of novelty was strong.
A few other interesting correlations emerged. Moderate coffee drinking increased the sperm count. On the other hand, men who a carried a mobile phone in a hip pocket or on their belt (but not if they carried it elsewhere) had reduced sperm counts: 65.6±1.26 x 106/ml vs. 75.67±1.30 x 106/ml.
All of these results were statistically significant.
We can learn some important lessons from this.
- Ditch the cell phone!
- Drink coffee—it makes you more manly.
- Don't look at porn too often, since you can habituate.
- When you do look at porn, the more explicit, the better.
- Here's the awkward one: the more studly guys lounging about in your porn, the more anxious your gonads will be, and the better their production.
Now when your girlfriend or wife finds you at the computer, coffee cup in hand, browsing some hardcore porn site, you can tell her that you are just trying to enhance your virility…and you have the statistics to support it. Of course, she's also going to be able to tell you to stop obsessively scanning through the two hundred porn portals you've got bookmarked—all things in moderation.
Kilgallon SJ, Simmons LW (2005) Image content influences men's semen quality. Biol. Lett. 1:253-255.
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Nice!
Read the paper, very interesting. Couldn't help but notice that the study was apparently done by two women who in the acknowledgements thanked the "Hollywood Fertility Clinic" for "training and support".
Training? Mental images abound, none of them printable!
But, PZ, that was all about humans not cephalopods! What if the porn contained hunky, competitive cephalopods though ...
... you're just going to have to run another study to find out. ;-)
On the more serious side, they didn't really take that habituation thing any further because it wasn't their immediate concern. However it is the concern of some other people when discussing sex crimes. Eg did people become unhabituated again over time? Did the subjects then crave harder porn? Did it affect their real life actions? As well as the brain influencing the testes, was the amount of effort the testes put in then influencing the brain back?
I am SO bringing this up in evolution class today. Another item of support for humans being shaped by male-male competition! They'll probably just gripe that unlike last week's killer catfish news, I'm not showing a video clip with this one...
I dunno.......this isn't very convincing to me. Aren't there other possible explanations for the reduced sperm motility in the test group? Like a subconscious, unacknowledged discomfort with homosexuality?
And before anyone goes on about how men always love to watch women together...believe it or not, not all men actually do like that.
You seem to be assuming that we all want more fertile baby gravy. Personally I'm taking away exactly the opposite conclusions.-Softcore only-Switch to decaf-Keep cell phone in tighty whiteys-No dudes in the pornThe wife'll be happy to know we can stop spending money on birth control now.
If it were discomfort with homosexuality, you'd expect the porn with guys frolicking in it would be more distressing. But sure, they need to repeat the study with more diverse porn: what is the effect of S&M, or scat, or foot fetish, or cephalopod porn on sperm counts?
One small nitpick: how can the quality result be significant, when the test group's motility rate was 52.1±7.3% whereas the control group's was 49.3±8.0%? Don't the means have to be outside each other's margin of error for the result to be significant?
Woohoo! Porn thread!
Here's some nice boobies:
(.)(.)
how is the motility result of 52.1±7.3% vs. 49.3±8.0% a statistically significant result?
beaten to the nitpicking punch!
Why not? Even if confidence intervals overlap, you can still have a significant difference in the distribution of the properties of two populations. The p value for that comparison was 0.034.
i really dig them boobies! but .. where were the calipers?
Is the '±' here referring to a margin of error? I took it to mean standard deviation, in which case there's no reason why it couldn't be statistically significant. Two datasets can lie within one SD of one another and still have a statistically significant difference between their means.
Mind you, 'statistically significant' doesn't always equate to 'significant'; if two populations are within one SD of one another, then the effect may be too weak to be of much practical importance.
Popular reports of scientific comparisons should start to include the p value regularly. Maybe then people would start to understand something about what statistical significance is, and what it does (and does not) mean.
This looks like an interesting read vis-a-vis the whole straight men reacting to lesbian porn thing:
Whitley, Bernard E., Jr.; Wiederman, Michael W.; Wryobeck, John M. "Correlates of heterosexual men's eroticization of lesbianism." Journal of Psychology and Human Sexuality 11, no. 1 (1999): 25-41.
Abstract:
If my old school has a subscription to this journal, I could try some IP-forwarding voodoo to get the article fulltext via a computer still on the school network. . . .
It makes sense for straight men to regard het porn that contains men as "pornier", because pictures showing just women are about the availability of coitus, whereas pictures including men are about actually having sex. It's like the difference between seeing food in the supermarket and seeing it being eaten.
Do they claim to support the specific evolutionary hypothesis that this difference in sperm quality is due to selection for sperm competition among males? That seems like a bit of a stretch with only this kind of data.
Yeah, about the "statistically significant" issue. Sure those means could be significantly different, given a large enough sample size (BTW this is not meant as a double entendre). But the real issue is how large an effect are we talking about here?
We can use these numbers to calculate Cohen's d, which if I recall correctly uses the formula d = (M1-M2)/s.d.
Given our numbers
M1 = 52.1
M2 = 49.3
s.d. = (7.3+8.0)/2 = 7.65
so d = (52.1-49.3)/7.65 = 0.366
As I recall, this size of d is considered a small-to-moderate effect size
But then the effect sizes for the other two effects are probably much larger. Not going to take the time to calculate them but judging from the diff between the two means in comparison to s.d. we are looking at BIG effects.
Forgot to add that this discussion of statistical effect size does not take into consideration whether the effect is of practical significance for the underlying phenomenon i.e. fertility.
I'm suspicious of that p-value. The correction for multiple comparisons suggests that a significance cutoff of 0.05 ought to have been adjsuted to ~0.025 (assuming 2 comparisons).
This is a marginally significant result, at best.
(But still interesting.)
I just have to say that there is definitely not as much discussion of porn and p-values on the internet as one might expect.
Doesn't it take something like 72 days for sperm maturation? I'm curious as to what kind of selectivity happens in the testes to provide "better" sperm if it seems to be warranted. Since it looks like they examined motility, maybe some kind of hormone surge during the final switch from sinusoidal tail movement to rotary? Anybody have any insight?
LMAO! Good one, PZ. I bet we won't see that one on CNN.com!
So, this started as a discussion about porn, and ended up on significance tests. You're just perverse!
Bob
BrassyDel:
Yeah it takes a while for sperm to mature, but there is some fine tuning at the last minute that affects volume of ejaculate and density of sperm in the ejaculate, plus which portion of the available sperm are released in the ejaculate (with younger/fresher sperm slightly more motile I would think) Presumably there are mechanisms which can control these variables at the time of emission.
I'm just glad we, as a species, can finally put the smackdown on those damn cell phones. Seriously, I've been dealing with those damn people when driving and it's good to know that not only are they Darwin Awards nominees, but now they won't give us more children that are just as dumb.
Yay, science!
Do they claim to support the specific evolutionary hypothesis that this difference in sperm quality is due to selection for sperm competition among males? That seems like a bit of a stretch with only this kind of data.
I think this is more supporting other data. I recall other experiments where in vitro, mixing human sperm from multiple males has resulted in sperm battles, where some sperm from each male tangle up with the competition, allowing other sperm to try to make it to the egg.
Anybody else hear about this?
Um... I think when men are caught looking at porn it isn't a coffee cup they have in their hand. Just sayin'.
I'll second polychrome's objection to your assumption that every man wants to be more fertile.
But, Matt, why then do they do things to increase their fertility? They may not be conscious of improving their fertility, but that's irrelevant. Evolution rewards improving fertility, conscious or not.
if two populations are within one SD of one another, then the effect may be too weak to be of much practical importance.
Heh. You said impotence.
dAVE: there have been many studies demonstrating the effect of sperm competition on male fertility and female choice. In fact, I know of several more that apply these ideas to speciation that are in the planning stages.
NelC: I'm certainly not claiming that you have to be conscious of an adaptation to make it one, in fact, I'm not even sure that makes any sense. I'm not taking issue with the claim that humans are a product of evolution, I'm taking issue with the evidence presented in this study. Is it enough to warrant the following assertion? ...our ancestors that were better able to increase sperm fertility when they suspected cheating had more offspring than those who did not increase their sperm's fertility. This is the claim that would seem to be supported (albeit very weakly) by the study as PZ portrayed it. Although he might have just added the bit about the sperm competition himself. I don't know, I haven't read the paper.
I am arguing that it seems like looking at some double penetration porn and jacking off in a cup is not a very good proxy for the complex social interactions that occur when dealing with infidelity. It would take more and different kinds of evidence to convince me that there has been selection for increased sperm fertility upon detection of infidelity.
Did they use a way of limiting the "output", though? It seems to me that, if their hypothesis were wrong, and men were more stimulated by the lesbian porn, they might have multiple outputs (ahem), which might reduce the overall count & quality (but increase the total quantity) as the "bottom of the barrel is scraped." I'd place a bit more trust in this if they limited their testing to first output, or categorized outputs separately, and perhaps did a comparison of how many outputs as well. But that might put some psychological pressure on the participants, I could see, that might skew results generally as well.
hey, i took my phone out of my pocket.
of course, i didn't have a phone when i was 19 and believed the whole "rythym method" thing.
The Economist had a piece on this some time back, though the study parameters were different: I think the bottom line of that one was that masturbation was a Good Thing, since it kept production levels up and rotated out stale stock regularly.
So for those want their output to less potent, don't even think about it, much less touch it.
"a higher percentage (52.1±7.3% vs. 49.3±8.0%) were actively motile."
The "±" may refer to the distribution of the motility and not of the average of the motility. Two averages may be significantly different even if the actual distributions overlap a lot.
at the computer, coffee cup in hand, browsing some hardcore porn site
Now THAT'S sperm competition!
So, when the Bible says, "be fruitful and multiply", it is really God telling us to look at porn. Who knew? Maybe there's something to this "Christianity" after all.
I wrote:
I'll second polychrome's objection to your assumption that every man wants to be more fertile.
Nelc replied:
But, Matt, why then do they do things to increase their fertility? They may not be conscious of improving their fertility, but that's irrelevant. Evolution rewards improving fertility, conscious or not.
NelC, I draw an extremely sharp distinction between what a person wants and what his or her genetics want. Every human being's genetic code wants that human to be more fertile, whether that human wants it or not. Your genome is not your friend. It would happily sabotage your plans and make you completely miserable in order to propogate itself at your expense.