I never said the number had to be positive. It's a complex topic, though, so I couldn't say with tremendous confidence that the number is negative - it's not a pure positional game, but has elements of positive, zero, and negative-sum games.
I would expect that the number of women (and men) who overinvest in attractiveness is positive. Ditto the number who underinvest. Both questions are interesting, imho.
http://theviewfromhell.blogspot.com/2012/09/toward-non-stupid-non-blank-slatey.html
The author gives a shout out to Less Wrong as a community with a perpetually skewed gender ratio, which is precisely the conditions under which polyandry appears to thrive.
Discuss. :)