In summary, we find that having sisters makes males more politically conservative in terms of their gender role attitudes and their partisanship. Particularly for gender role attitudes, we find that these political socialization effects persist until respondents are well into adulthood. Since sibling gender is randomly assigned, we can interpret our results as causal evidence that the household environment (cleaned of genetics, the external environment, social forces, and other such omitted variables) influences political attitudes.
Source: Healy, Andrew and Neil Malhotra. 2012. “Childhood Socialization and Political Attitudes: Evidence from a Natural Experiment,” (revise and resubmit, Journal of Politics). You can find a link to the paper here. See other papers by Andrew Healy here.
Imagine the implications for the political future of China . . .