The teenage slut gene

NewScientist mag reports:

Sexual precociousness is in our genes, new research suggests. A unique study of twins separated at birth finds a genetic link to the age at which a person first engages in sexual intercourse. ...

However, determining the extent to which sexual precociousness is inherited is trickier than making a similar calculation for height. A common family environment – whether it promotes or hinders early sex – could cause scientists to overestimate the effect of genes.

Social effect
By studying 48 pairs of twins raised apart, as well as 23 individual twins, Segal's team sidesteps this confounding factor. "This gives us a pure estimate about how much genes affect behaviour," she says. ...

Segal's team also found that female participants who felt unhappy and unfulfilled in their home life were more likely to have sex at a younger age.

The estimate cannot possibly be that pure. There could be a gene that causes a girl to be "unhappy and unfulfilled" if she also has a lousy home life. Most behavior is probably partially genetic, but pure estimates are nearly impossible.