A new study in rats says growing up with lots of sisters makes a male less sexy to females, because the sex ratio of a male rat's family while young influences his sexual behavior and therefore how female rats respond to him later.

Early life obviously affects later behavior but how early and how much of an impact and how can it be quantified in a less 'soft' fashion.    There are correlation studies that even conclude the position of a fetus in the uterus matters and that a female fetus that spends the pregnancy sandwiched between two brothers grows up to be more masculine because she's been exposed to their hormones.    

What about the family situation after being born?   In the new study, researchers counted the number of males and females in each litter to determine the sex ratio in the womb. Then they reassembled litters in three ways: so the litters were balanced between males and females, strongly male-biased, or strongly female-biased. Then they observed the mother's behaviors toward their pups and, once the males grew up, tested them to see how they behaved with female rats. 

The researchers found no effects of the sex ratio in the uterus but they did find differences in behavior based on the kind of litter in which the males grew up. 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, which means the males with higher female ratios were more efficient at mating, just less fun.

The males may be compensating for the fact that they're less attractive to females, the researchers hypothesize. 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
David Crews, Ashbel Smith Professor, Section of Integrative Biology and Department of Psychology at at the University of Texas at Austin, 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. 

These were rats, but the results may have implications for humans, too, Crews says. "It 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." 

Citation: Cynthia B. de Medeiros, Stephanie L. Rees, Maheleth Llinas, Alison S. Fleming, and David Crews, "Distinguishing the Contributions of Prenatal and Postnatal Factors to Adult Male Sexual Behavior in the Rat, Psychological Science, October 2010; vol. 21, 10: pp. 1494-1501 doi: 10.1177/0956797610382122