[cosmie]

From my interpretation, that wasn't an assumption of the theory. All else being equal, there is presumably a similar distribution of intelligence among attractive females as there is among less attractive females.

However, due to attractiveness having a heavier weight than ability in the hiring decision, you hire disproportionately from the full distribution of attractive female candidates, rather than just the "above-average" sides of both distributions. This leaves an artificially scarce supply of female employees competent enough for upper-management level.

In contrast, if men are hired more on competency and less on looks, then you're not arbitrarily splitting your candidate pool into two distributions - one of which (the unattractive) you are very unlikely to hire from - and you're likely to hire all competent males vs. some competent and a few that really shouldn't have been hired but looked good.