This could explain a lot.
A new study suggests boys who grow up with only sisters are more likely to be Republicans as adults.
The research, published in the new Journal of Politics, found that boys with only female siblings were 9.3 percent more likely to identify as Republicans than those with only brothers.
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They're also less likely to help with chores when they grow up.
The reason, professors at Stanford and Loyola Marymount universities say, is because boys who grow up with only sisters tend to develop more traditional views of gender roles in society.
Women stay home and cook, clean and do the laundry. Men bring home the bacon.
“Having sisters makes males more politically conservative in terms of their gender role attitudes and their partisanship,” study authors Andrew Healy and Neil Malhotra wrote. “Particularly for gender role attitudes, we find that these political socialization effects persist until respondents are well into adulthood.”
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The "gender effect" is stronger for boys who are the oldest in the family and those who grow up close in age to their sisters, according to the study.
Growing up with only sisters had no impact on girls in the study.
The analysis was based on surveys of more than 3,000 individuals conducted in 2006 and 2008 as part of the government's ongoing National Longitudinal Survey of Youth.