This looks like one of those questions that pops up when you start typing a query into a Google Search Box, but it is really a question asked rhetorically by Claudia Sawyer on my facebook status where I made mention of the fact that toddlers will put anything you give them in their mouth (even after that “putting everything in their mouth” phase is over … they still do it enough that you can’t give them knives or sandpaper, believe it or not).

And there actually is an answer to this question and it comes from science. The answer is about five years old, and here’s why:

A newborn mammal requires a huge amount of care. Care that might be provided by parents that is siphoned off for any reason is crucial, and the more that is siphoned off the less chance the infant has to survive, or grow strong, become a pride leader or head giraffe, or get into a good school, or whatever, depending on species. From a purely selfish Darwinian point of view, the offspring, as it grows through life, should continue to find more and more effective ways of getting energy and attention and protection and so on from caregivers.

The parents, however, want to reduce investment in that child over time and eventually start investing in offspring N+1, again for purely selfish Darwinian reasons (to have more offspring). Parents in species where there is an extended required period of care, like humans, should be selected to forgo this investment child N+1 while child N sill requires a lot of care, but at some point should, in a purely Darwnian world (where all the calculus is Darwinian) shift investment to child N+1.

The question is, when do you switch? Since the child is less related to the parent than the parent is to itself, there may be a bit of conflict of interest there. Also, child N+1 is more related to the parent than child N+1 is to child N. From the point of view of “kin selection” which takes into account not only one’s own offspring, but also part of some relatives (depending), it the child N sees child N+1 as less valuable than parent sees either.

Even at the purely genetic level this actually gets quite complicated because in order for a behavioral thing to happen “adaptively” there has to be an effect. Up to this point we have been speaking pretty much of potential strategies or “desires” from a formulaic point of view. In order for any of this to matter, the child N or the parent mush be able to do something that affects the reproductive and care-giving strategies in question. For example, a child can increase or decrease demands on the parent or the parent can force, convince, or train a child to do something differently. When child N is old enough it can help take care of child N+1 like many birds do or, as is the case hyenas, child N can kill child N+1 (where the two are born as litter mates but one is still N and the other is still N+1).

So, why are toddlers always trying to kill themselves and when does it stop?

Both child N and parents want child N to get enough care, and both also want child N+1 to have enough care. But because of the genetical asymetry between parent and child N+1 vs child N and child N+1 (there is a much higher chance of shared genetic material for the former than for the latter), the calculus for these dyads is different.

Putting it a slightly different way both parent and offspring N want parent to start investing in offspring N+1 at some point, but parent wants to do it sooner and offspring N wants to do it later.

And this could be part of the explanation for why infants and toddlers do some of the stuff they do. Blackmail, interference with reproduction, resource hogging, etc. is observed in many species (baby pelicans biting their wings unless they are fed comes to mind). All of this could be explained by a simple conflict between parent and offspring without consideration of offspring N+1 but Robert Trivers makes a pretty good argument that the parent to child vs child to child asymmetry should explain certain things, in a simplified neo-Darwinian (that’s Darwin plus genes) world. You can get access to the paper Trivers covers this in and other works in Natural Selection and Social Theory: Selected Papers of Robert Trivers (Evolution and Cognition).

Comments

  1. #1 Daniel J. Andrews
    June 28, 2012

    So, why are toddlers always trying to kill themselves and when does it stop?

    It stops when toddlers are no longer toddlers. The propensity to stick dangerous things into their mouths may continue well into adulthood although it seems to reach a peak during the 18-24 year old stage (think alcohol consumption). After that, it tends to decline because 1) they’ve been successful in killing themselves, and 2) they mature and realize pickling themselves on a regular basis isn’t really all that fun, never was, and the only reason they did it was because everyone else they associated with was also doing it—and tv commercials, university culture all make it seem like the thing to do. And all the sheeple said “baahhhhhhh”.

    Snark aside, interesting ideas by Trivers–not sure how well they play out aside from selected examples. Still, intriguing possibilities there.

  2. #2 Greg Laden
    June 28, 2012

    Trivers’ Parent Offspring Conflict theory sparked an entire generation of research so there are actually hundreds of case studies among various animals. The problem is that life in general is so complex that it remains hard to sort out.

  3. #3 Tamara
    June 28, 2012

    Aren’t child N and Child N+1 on average related to each other as much as they are to either parent? They share half of their DNA with each parent (plus or minus a little due to sex chromosomes) and an average of half of their DNA with each sibling, sometimes considerably less, sometimes considerably more. Unless I misread my Dawkins…it’s been a very long time, I’m embarrassed to say.

    So, a test of this theory could be to see if, under controlled circumstances, children with younger siblings engage in more risky behavior than do only children of the same age.

  4. #4 Tamara
    June 28, 2012

    And suddenly I realize that I made a big assumption: That child N and child N+1 always have both parents in common. My bad there.

  5. #5 Greg Laden
    June 28, 2012

    That is part of it. But mainly, the parent is weighing taking resources from a 0.5 and giving it to a 0.5 while offspring N is weighing resources being taken from a 1.0 (self) and being given to a 0.5 (or 0.25).

  6. #6 Vince Whirlwind
    June 28, 2012

    Got to take exception with your equivalence between, “…to survive, or grow strong, become a pride leader or head giraffe,…” and “…get into a good school…”.

    I’d say getting into a good school has an inverse relationship with reproductive fitness.

    Hence less and less of us are paying tax, while more and more of them are living off welfare and breeding like rabbits.

  7. #7 Greg Laden
    June 28, 2012

    You’re right. I meant “avoid grad school”!

  8. #8 Ian Kemmish
    June 29, 2012

    One suspects that the answer won’t be entirely given by genetics.

    On the one hand, you have young children in rural parts of Africa left to look after precious resources like the family’s flock and allowed – expected – to handle fearsomely sharp knives.

    Then you have mid-20th century Western countries, where children discovered that knives were tools just before puberty and spent all their time making models of things.

    Then you have contemporary Western cultures, where from puberty up until the birth of the first child (sometimes beyond, if the country has a good welfare system), they indulge in tombstoning, experimenting with drugs, gang warfare (including terrorism, of course) and so on.

    Then you have BDSMers, up to half of whom never stop trying to kill themselves….

  9. #9 Greg Laden
    June 29, 2012

    Those are all interesting points some of which were on my mind as I wrote this. Having lived in Rural Africa for several years, I can tell you that the knives are not fearsomely sharp! But little kids do have access to things like machetes etc that would kind of freak out westerners.

    But this brings up an entirely different but parallel question, about context.

    In my view, western toddlers are living in a state of great discordance between pretty much any hypothetical “EEA” and their actual surroundings. Even if one postulated (this would be wrong, but that won’t stop people from thinking it) that we evolved to be repulsed by natural poison things (berries, other plant product, etc.) we live in an environment where the poisons are found in attractive bottles with happy people (or Mr. Clean) depicted on them. I once helped rescue a kid who had eaten an entire bottle of blood pressure pills. He lived (it was close) but what he did was fairly logical. Bright blue, pretty, sugar coated. Of course he ate a bottle of them.