Via Melissa Lafsky at the Freakonomics blog,
A working paper on gender differences in competition by Uri Gneezy, John List, and Kenneth Leonard:
Our experimental results reveal interesting differences in competitiveness: in the patriarchal society women are less competitive than men, a result consistent with student data drawn from Western cultures. Yet, this result reverses in the matrilineal society, where we find that women are more competitive than men. Perhaps surprisingly, Khasi women are even slightly more competitive than Maasai men, but this difference is not statistically significant at conventional levels under any of our formal statistical tests.