Alfred Adler, a contemporary of Sigmund Freud, was among the first professionals to suggest birth order shaped personalities. In subsequent years, the theory has become a controversial issue in psychology. Now it seems that even the gender of one’s siblings might also have a long-lasting influence. As the Association for Psychological Science reports, a new study from the University of Texas, Austin, shows that males who grew up with an abundance of female siblings react differently in sexual situations than males who grew up with balanced or overly male families.

At least in rats, anyway. By studying several families of rats, researchers were able to determine a difference in male rat behavior by its birth. David Crews is a psychobiologist at the University of Texas at Austin and the leader of the study. Crews and his team found that the position of the rat fetus in utereo influenced its later behavior. For example, if a female rat is sandwiched between two male rats, it will be exposed to more testosterone. Therefore, it will grow up to be more masculine.

But it is also the sex ratio of the litters that can have a profound effect on the behavior of its occupants. Researchers looked at litters that were balanced between gender, strongly male, or strongly female. It was found that the male rats with more sisters spent less time actually having sex when presented with receptive female rats. However, they were more efficient at getting the job done.

Researchers believe this is because these male rats are compensating for the fact that they are considered less attractive to females. Crews notes that if female rats want to mate with males, “they wiggle their ears. It drives males nuts.” They also hop back and forth. As Crews and his team found, female rats exhibited less of these behaviors with male rats that had a lot of sisters.

Ultimately, Crews thinks his research has implications on human families as well. According to Crews, “It tells you that families are important — how many brothers and sisters you have, and the interaction among those individuals.” 

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