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I was amazed when I started doing this research how many studies there were that actually
looked at comparing cognitive navigation ability comparing males and females. There have been
a number of meta-analysis; meaning there are studies that have actually looked at all the
other studies and collected data showing, very conclusively, when you test male ability
on these navigation type tasks, they perform better.
This does not mean that every male outperforms every female. There are huge distributions.
Just, on average, males slightly do better than females on this. So you see a very well
characterized sex difference in cognition. And then stories are told about how this is
an adaptation. They tell the story about the Hunter-Gatherer.
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Because the men were the hunters and they needed to find their way back, only the men
that found their way back had babies. And those are the men that had the genes for good
navigation. Therefore, genes for male advantage in navigation would evolve. That is the story.
But that does not necessarily mean that that story is correct. So we need to bring evidence
to the table to try to support that story. If you think
about it though, a little bit more deeply, you realize that there are some major holes
in that story to begin with.
If you think about the story, only the males that were able to make it back passed on their
genes. However, remember that those males pass on their genes to their female offspring
as well. So the female offspring are also going to inherit those genes for superior
navigation.
The way to think about this nipples for example. Notice that males also get nipples. Why is
that? Because the genes that lead to nipples will also be passed on to male offspring because
nothing is going to prevent them. Since nipples do not harm males, selection is not going
to differentiate them and so men have nipples.
The only way you can force a trait to be under the control of the Y-chromosome, in a sense,
is by making what is good for one sex, bad for another. That is what will lead to such
strong differences in sex dimorphism. It is hard to imagine how you would have antagonistic
selection in the case of navigation. So it is necessary for the story to explain why
enhanced or superior navigation was bad for females. Otherwise, it would have been inherited
by the females as well. So that is the first problem with that idea.
The second problem is that, if the hypothesis that everyone is buying into is correct, that
the reason that males have the superior performance is because they were the hunters and that
gave them the advantage, then we should be able to test that hypothesis by looking across
species. Those species where the males range relatively farther than the females should
show relatively greater spatial navigation. Whereas species that show relatively similar
home ranges, should show relatively similar level of performance. So we actually did this
across eleven different species we were able to find from the literature.
There were a number of species of rodents, rats, mice, different deer mice, horses, and
different non-human primates, and different vole species. So there have been eleven species
we were able to collect. And, if you look at the data, what you see is that, in fact,
there is no positive correlation at all. And across all the species generally, irregardless
of what the size difference is in the home range are between the sexes, males tended
to have an advantage on these navigational tasks. And so, if all these other species
show the same sex differences as humans, it seems very unlikely that this is something
that evolved along a human lineage.
Now that is exactly what you would predict based on an alternative hypothesis that I
suggest. So, what if it is not an adaptation and it has no functional significance at all?
It is just a side effect of testosterone. Meaning that testosterone has evolved to in
males to produce some very important male characteristics -- namely making penises
and whatever other secondary sex characteristics, like muscle size or body mass or behavior
that makes men successful males in breeding with females. And note that testosterone is
an adaptation for producing those characteristics. But anything else that testosterone does,
as long as it does not hard male success, constitutes a solution.
For example, there are many side effects of testosterone that we know about. For example
male-pattern baldness, like acne that occurs during adolescence, like the fact that your
ring finger is longer than your index finger and that that difference is associated with
amount of testosterone. So these are just some examples. Those do not serve any functional
purpose in males. They are just side effects of testosterone. And one more side effect
is a slightly different reorganization of parts of the brain involved with navigation
that give males a slight advantage.
The data produce greater support for what I call the testosterone spillover hypothesis.
And they do not really support at all the adaptation hypothesis; in the sense that the
correlation was not positive, it was flat.
The impact of this paper is that, “Look! If you actually go and do the test, you find
that the data actually support the alternative hypothesis. So maybe we are fooling ourselves
into thinking that some of these cognitive traits really have functional significance
when, in fact, they are nothing more than a spillover, or what people refer to as a spandril.
Spatial navigation ability could have been slightly better in females than in males.
There is a fifty-fifty chance. In this case, males seem to be better. But, if you look
at a different trait, it could be the opposite. In fact, look at other traits and you will
see the distributions are shifted, where females have a slight advantage. Is that always, in
every instance, in each sex difference are these shaped by natural selection? Might this
be just a side effect or a spandril or a consequence of the hormones that are not directly functionally
relevant? I think, when you look at the whole thing and think carefully about it, you conclude
that, yes, it probably is.
So the big conclusion is that maybe we should not be so to just accept the adaptation stories.
Maybe there actually is a place for side effects and spandrils with respect to cognitive performance
differences between the sexes and other stories that are told about human behavior that may
not really be adaptations, but alternative explanations that people really have not considered.
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