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Originally Posted by Delvo
The answer is that it can go either way. Evolution uses whatever mutations come up, and a mutation for increasing mathematical intelligence could come up in a form that is sex-linked, or in a form that is not. (And we already know that fetal development of the brain is mostly the same for both sexes but does have some little differences in response to sex hormones, so examples of both sex-linked and sex-unlinked genes for something about the brain are already established.) Sometimes the same trait that was sex-linked at first can even become unlinked, or the other way around, if the creation or breaking of the link is beneficial in selection. For example, in Asian elephants, having tusks is a male trait, but in African elephants, both sexes have tusks. So whichever way it was in their last common ancestor, it had to have changed (becoming sex-linked or losing that link) in at least one of those species since then.
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That's a good uncontroversial example. In most birds and mammals there are many physical differences between the sexes apart from the reproductive organs, and also differences in behavior. Hardly anyone expects when encountering a new species of bird or mammal that the sexes will be nearly identical in form and behavior.
Quote:
Originally Posted by Delvo
If we used tests to measure something about one sex or the other in our own species as you describe, so it only really matters to one sex, then the other sex would still be free to go along for the ride and be affected by the change, or not. In fact, I can give an example of it, or something like it, that doesn't seem to be connected to brain action (at least not very directly). Humans and our ancestors, over the last several million years, have kept getting more and more lightly built, with narrower bones, less muscle mass as a percentage of total mass, and smaller tendon attachment sites on the bones. Given the obvious differences between male & female skeletons & muscles, I'm sure you can see that this means we've been getting more "feminine" (at least as femininity is defined in this lineage; lions would agree, but bears and eagles would not). But it includes the men getting that way over time too!
There are only a few possible explanations. If it benefits females significantly more than males (and might not benefit males at all, as it seems to actually be detrimental), then males are just being dragged in that direction as a side effect. If it benefits both sexes about equally, then some quirk about the biochemical mechanism by which it works just happens to make it affect females more strongly than it affects males, as a side effect.
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It's been speculated that the selection pressure was on the males because females preferred more feminine mates. Whether this is true today I leave to those more familiar with female psychology.
A more boring explanation is that when food is scarce (protein in particular) growing huge muscles is expensive and at some point the return from investing in brain development became greater than from investing in muscle development. Being able to make spears and throw them accurately, to build traps, and to use sophisticated hunting strategies may make you a more effective hunter than having huge muscles and relying on running down and beating animals to death with you bare hands.
Hunting may be over-emphasized. Gathering may have been more important than hunting. Obviously brains make you a better gatherer too, and huge muscles are less useful to a gatherer than a hunter.