Your siblings shape your development in all sorts of ways. You play with them, fight with them, learn from them, and probably ask them inane questions your parents are too tired to answer. They’re often the peers you spend the most time with growing up, and being the oldest or the youngest certainly plays a role in finding your place in the family dynamic. Your birth order and your sibling’s gender might also influence your personality. 

For men, having an older sister can decrease the likelihood of being a competitive person, according to a recent study in the journal Personality and Individual Differences by Okayama University economist Hiroko Okudaira. Compared to men who were only children, men who had older sisters (but no older brothers) weren’t as interested in competing for prizes and money in a series of tasks across two studies of Japanese students.

In the first study, 135 Japanese high school students solved mazes in exchange for points that could be traded for prizes. Before the task, they had to select whether they would rather get points according to the number of mazes they solved, or be entered into a tournament against other participants, in which you had to solve more mazes than the rest of the group to get points (a competitive environment). Only 38 percent of men who had an older sister entered the tournament, showing a lower preference for competition than men with no older sisters.

In a follow-up study, 232 university students solved math problems for cash, with the same tournament setup. Only 24 percent of men who had older sisters entered the tournament, compared to 48 percent of the rest of the men. 

The second study is limited by the fact that it studied only university students at Osaka University, a very selective college, thus increasing the likelihood that the participants would already have a preference for competition. However, psychologists have long speculated that having siblings of the opposite sex can affect the gender-stereotypical traits you exhibit, and this provides further evidence for that claim. 

Why would having a sister influence your personality? Having an older sibling of the opposite sex has been shown to decrease the stereotypical gender roles you inhabit. Competitiveness is a stereotypically male trait, influenced by testosterone, so having an older sister may mediate that characteristic in boys. However, women with older brothers were not significantly more competitive in either of the tests. Here, birth order may also play a role, as later-born kids are generally less dominant than their first-born siblings.

Sorry guys, but it seems like your bossy older sis will still be controlling you long after you outgrow childhood spats.

[h/t: BPS Research Digest]