A longitudinal investigation of the social and cognitive development of male twins was conducted when twins were 2.5 years of age, and again when they were 8- to 10-years-old. This study was designed to re-examine the heritability of the traits studied at the earlier age and, thus, to address the question of the stability of genetic determination. The sample of the original study consisted of 46 sets of male twins, 17 of which were monozygotic and 29 dizygotic. At follow-up, 35 pairs (13 monozygotic and 22 dizygotic) participated. The earlier study investigated the genetic determination of social characteristics, speech, and vocabulary intelligence quotient. The follow-up focused on genetic and environmental factors associated with the same characteristics, as well as antisocial and neurotic behavior, nonverbal ability, school achievement, height, and weight. At the age of 9 years, significant genetic variance, replicated by two methods of calculation, was found in height, weight, nonverbal intelligence quotient, and antisocial behavior. Shared family environmental influences were important for compliance, conscience, and antisocial behavior. Social traits were thus under less genetic influence than were physical and cognitive characteristics. Both genetic and environmental factors were important for antisocial behavior. Stable genetic determination across earlier and later ages was found only for speech. (Author/RH)