The ratio of male and females within a family growing up together can influence the sexual behavior.
Having a large number of female siblings makes men no less heterosexual but their mannerisms and body language may be seen as less butch than those who have been brought up amid the rough and tumble of a male dominated household.
Researchers discovered the ratio of male and females within a family growing up together can influence the sexual behaviour, rather than the sexuality, of a boy who is outnumbered.
Genes may determine personality and behaviour but the environment during early life “shapes it” said the study published in the journal Psychological Science.
A team of psychobiologists from the University of Texas experimented with new born rats, separating them into litters either male dominated, female dominated or equally mixed.
They found sexual behaviour was influenced more by the male-female ratio of the litter in which they were brought up than the one to which they were born.
When it came to mating, the male rats brought up in a litter of mainly sisters, spent less time mating than those brought up among male rats or in an equally divided litter.
This was because they were not being invited to do so by the females who signal their availability by wiggling their ears or ‘dart hopping’ – an established rodent come on!
David Crews, the report author, said: “If they (females) want to mate with a male, they’ll do a move called a dart-hop.
“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.”
And what applies to rats may have implications for humans too, he added.
“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. The environment where you were raised doesn’t determine personality, but it helps to shape it.”