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) .