Environmental and genetical causes of variation in ethological aspects of behavior in two‐year‐old boys

Abstract

The interactive behavior of two‐year‐old male twins (17 MZ, 29 DZ pairs) with their parents was observed in the home and laboratory. Behavior counts, ratings, and experimental measures were subjected to a biometrical genetic analysis. A model which included only within‐and between‐families environmental effects fitted most variables best, except for Instrumental Independence (a rating) and Speech Rate (a count variable) which showed a significant genetic component. Low reliability may explain the small proportion of genetic variation in count variables, but the greater reliability of ratings and experimental measures (the latter, however, of doubtful construct validity) did not produce greater genetic determination. The largest part of the variance was explained by between‐families environmental differences’, only a minor part of which was due to interobserver differences. The probability of detecting genetic determination with this sample size, although it was large for an ethological study, is rather low.