Well, for rats at least. In a study done at the University of Texas at Austin, researchers found that male rats who come from female-dominated litters are less sexually aggressive and less attractive to lady mice later on in life.
The study involved shuffling newborn litters into groups by gender—some groups were mostly male or female, while others were equal—and then observing their mating behavior once they matured. Results showed that while male rats who’d grown up with a lot of sisters were just as physically vital, they spent less time pursuing and being pursued by potential mates.
When males who were raised with a lot of sisters were presented with receptive female rats, they spent less time mounting them than did male rats that were raised in male-biased litters or in balanced families. But they penetrated the female rats and ejaculated just as much as did the other males.
(Fun fact: When female rats are interested in mating, they do something call a “dart-hop,” which the researchers claim “drives males nuts.” It’s how humans can tell whether a guy rat’s got game.)
So does this translate to humans? Not exactly. While sibling studies have shown that having sisters could prevent depression—and having an older brother often sparks more aggression—there’s no direct correlation between our species … yet.
As lead researcher David Crews puts it, “[This research] tells you that families are important—how many brothers and sisters you have, and the interaction among those individuals.”
Families are particularly important in shaping personalities, he says. The environment where you were raised “doesn’t determine personality, but it helps to shape it.”