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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 study by 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.

Perhaps even more important than the impact sisters have on men’s political views is the way sisters may influence how their brothers turn out as husbands. The study found that boys with sisters grow up to be men who don’t help much around the house. The researchers’ data show that middle-aged men who grew up with a sister are 17 percent more likely to say their spouses did more housework than they did compared with men who had only brothers. The study suggests this might mean men’s views of gender roles are permanently affected by their childhood environment. Girls weren’t affected by having brothers or sisters.

The results seemed to surprise the researchers, who thought having a sister would have a liberalizing effect on boys. Loyola’s Andrew Healy said in a press release about the study:

We might expect that boys would learn to support gender equity through interactions with their sisters. However, the data suggest that other forces are more important in driving men’s political attitudes, including whether the family assigned chores, such as dishwashing, according to traditional gender roles.

Message to parents: If you want your boys to grow up to be good husbands or partners, make them wash some dishes and iron clothes when they’re young!