Too many sisters 'make a man less sexy'

Too many sisters can make a man a lot less sexier, at least in case of rats, according to a new study from the University of Texas at Austin.

"Life is a continuous process: you're a fetus, then you're born into a family. Each one of these periods can be important" he says—"and they don't necessarily have the same effects" said David Crews, a psychobiologist.

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.

This means, "the males are more efficient at mating" Crews said.

The males may be compensating for the fact that they're less attractive to females. You can tell this by watching the females—if they want to mate with a male, they'll do a move called a dart-hop, says Crews, and they wiggle their ears. It drives males nuts."

The females did this less when they were with a male rat that had grown up in a female-biased litter.

"It tells you that families are important—how many brothers and sisters you have, and the interaction among those individuals," Crews said.

The environment where you were raised "doesn't determine personality, but it helps to shape it."

The new study is published in Psychological Science.

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