From Mother Jones:
Lots of people have been looking to science to explain the differences between Democrats and Republicans. Mother Jones‘ Chris Mooney has published a rundown of all the brain differences suspected in the gulf between liberals and conservatives. But a new studyby researchers from Loyola Marymount University and Stanford University’s business school suggests another factor may play a role in forming the political brain: the gender of one’s siblings. According to the study, boys with only a sister were 15 percent more likely to identify as a Republican in high school, and they were 13.5 percent more conservative in their views of women’s roles than boys who only had brothers.
The reason for this difference? Not genes or neural pathways, but something more mundane: housework. The researchers speculate that boys take their cues about women’s roles from an early age, and that girls tend to be assigned more traditional chores when they have a brother. Watching their sisters do this housework “teaches” boys that washing dishes and other such drudgery is simply women’s work. Boys with only brothers don’t seem to have this problem because the chore load at home tends to be spread around more equally. The impact on men’s gender perceptions is long term, but the stark partisanship fades somewhat as men get older, the researchers say.