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Dr. Laurie Santos is an Associate Professor of Psychology at Yale University. Her research provides an interface between evolutionary biology, developmental psychology, and cognitive neuroscience, exploring the evolutionary origins of the human[…]

There are morphological indications that we’re somewhere between a species meant to pair bond and our polygamist evolutionary relatives.

Question:  From an evolutionary perspective, are humans rnnaturally polygamists?

Laurie Santos: The human reproductive system is one rnthat we don’t really have a great grasp on because in some sense we pairrn bond, for the most part, but across all human culture, there’s really arn push to polygamy. So, in most human culture there are at least some rnmales who take on multiple mates and have, you know, multiple mating rnpartners.  But not nearly to the degree that you see in chimpanzees.  rnThey way you can tell this morphologically is by the size of the testes rnrelative to body size. So a chimpanzee's testicles relative to body sizern are just like enormous. You would blush to see the size of these rnthings.  Not nearly the same ratio as you see in humans.  However, rnhumans have a larger testicle to body size ratio than you might see in rnother primates where we know that the females don’t kind of sleep aroundrn as the case of gorillas. 
So, the human mating system is kindrn of somewhere in between. We’re sort of pair bonded. There’s this push rnto polygamy, there’s a push of males taking on multiple female partners,rn but there also seems to be a push toward polyandry. In other words, rnfemales taking on multiple male partners, or else why would males kind rnof grow these big testicles to kind of compete at the level of sperm.  rnSo, we're in this funny puzzle, in terms of why humans might pair bond. rn One of the pushes towards pair bonding in the animal kingdom has to do rnwith the kind of size and cumbersomeness of your off spring.  So, the rntaxo where you see the most pair bonding is in birds.  I mean you can rnsee this in kind of the standard, sort of “March of the Penguins,” wherern the two parents you know, very cutely take care of the kids. But it’s rntrue in birds because the offspring actually require a lot of work. rnThere’s this extremely fragile egg, you know, that’s very tasty and you rnhave to defend it from predators and so on.  And they it actually rnrequires both parents to actually incubate the egg, you know, protect itrn and so on. 
The idea as they say might be true of humans.  rnYou know, human infants are born incredibly precocial. So human infants rnare born incredibly precocial, much more so than you know, other close rnprimate relatives, you know, they’re pretty fragile.  You know, if you rnjust left a human baby newborn there for a long time, you know, it rnwouldn’t do so well.  The thought is maybe this human pair bonding rnactually came as a result of the fragileness of human infants.  You rnknow, in that it might require two parents to actually take care of rnthese offspring. 
But again, these are... there are a lot of rnjust so stories out there, you know, it really hard to figure out rnexactly why we have the reproductive system we have and it’s a rncomplicated one that we can’t pin down.  The sad thing that I mentioned rnin my course is that we know much more about the reproductive systems ofrn pipe fish and swans and lions, then we do about our own species.  Whichrn is kind of pathetic.

Recorded May 21, 2010
Interviewed by Andrew Dermont