Economists and other social scientists typically rely on gender differences in the family-career balance, discrimination, and ability to explain gender gaps in wages and in the prospect for advancement. A new explanation that has recently surfaced in the economics literature is that men are more competitively inclined than women, and having a successful career requires competitiveness. A natural question revolves around the underlying determinants of these documented competitive differences: are women simply born less competitive, or do they become so through the process of socialization? To shed light on this issue, we compare the competitiveness of children in matrilineal and patriarchal societies to show that the difference starts around puberty. Moreover, most of the changes during this period of life are within the patriarchal society, in which boys become more competitive with age while girls become less competitive.