Stay with me here, tell me where my logic is faulty.

Statements of the obvious:

1> At the earlier stages of human evolution, brain volume increased
very very rapidly, in a manner which is almost universally believed to
be a result of intelligence-based selection, possibly sexual selection
driven by the complexities of comparatively larger human tribal size.

2> Significant infant and mother mortality was associated with the
generally larger infant heads.

Imaginative Leaps

3> Females generally have smaller heads than males, and therefore it
is possible that at an earlier stage in human evolution, many more
male humans died during childbirth than women, leading to a time when
female humans outnumbered male humans based on a differential in head
size.

4> Furthermore, there is a significant possibility that female humans
of this period could have evolved to have proportionately larger heads
than male humans. This is because a 5% increase in male head size
might (say) double infant mortality, where as a 5% increase in female
head size might not have the same associated rise in infant mortality.

5> Therefore it is possible that for a very, very long and intense
period of human evolution, women on-average had proportionately
greater cranial volume increases than men.

6> If cranial volume increased human mental capacity, and at this
time, the strongest suggestion is that it did, then for a long and
critical evolutionary period, women were smarter than men.

This might go part way to explain why women appear to casual observers
to have a larger Dunbar number than men.

7> One genetic pressure resulting from this selective pressure is for
male humans with proportionately smaller heads to survive, where
larger-headed babies died more frequently.

9> Females typically have smaller heads than males, as stated before.

Wild Speculation

10> There might have been an adaptation in which some male humans
followed a biological curve closer to the female curve while in the
womb. The selective pressure towards this would be smaller head size
resulting in reduced infant and mother mortality. One possible
mechanism to produce this developmental change would be a change in,
say, hormone levels leading to the acquisition of some female
characteristics in the womb, including smaller heads.

11> The result might have been intelligent men originally had strong
female characteristics.

12> Hence, homosexuality could have evolved – both genders could
prosper better when some proportion of the male embryos had a
closer-to-female developmental curve, resulting in proportionately
smarter human males who had an increased probability of female
characteristics in other areas, because of the developmental changes
in the womb resulting in the imagined smaller male head size.

The Crazy Part

* This would suggest that some of the human archetypes (dumb ox-like
hetro-sexual males, and correspondingly intelligent, articulate,
social homosexuals, and also women who are generally smarter than men)
relate to a previous time in human evolution, and we expect them to
exist in the environment due to genetic factors that relate to a much
earlier stage in our evolution, when these types of human were
predominant in the environment.

* It might have been that male homosexuals were very genetically
different from male non-homosexuals at one point in time, but as
intelligence increased and infant mortality at a given head size
dropped due to co-evolution of larger pelvis, the genes for
intelligence might have circulated freely, keeping the human race at
two genders.

Flaws in the chain of argument

A> Does the fossil record indicate there a time when female craniums
were proportionately larger to male craniums than now?

B> In cultures without extremely strong medical care systems, does
infant mortality vary with head size? Or is there evidence that it did
so at one time in the fossil record?

Logical problems

C> Wouldn’t the male humans with partial female characteristics derive
great benefits from being as active in breeding as the hypothetical
straight-male humans? Why didn’t that evolve? Did it?

D> What about homosexuality in non-human mammals? Is there a similar
head size story there?

If there no human-like history of head size selective pressure in
other mammals, which still have significant homosexuality rates, then
this is dead theory. And I’m fairly sure there’s no such history.

Thoughts? Can anybody see a way of fixing that fatal flaw in this theory?