a1 Department of Genetics, University of Birmingham, Denmark Hill, London
a2 Department of Psychology, Institute of Psychiatry, Denmark Hill, London
A study of MZ and DZ twins suggests that cultural differences are less important than individual environmental experiences in determining the age of first sexual intercourse. Certain personality factors are found to be correlated with this trait and there is also evidence that genetical differences may predispose individuals to cross this threshold at an earlier or a later age.
(Received June 21 1976)