Of biases and more in the study of twins reared together: a reply to Grayson.

Grayson (see the preceding paper) discusses some circumstances in which estimates of genetic and environmental parameters derived from the study of twins reared together may be biased and documents in those circumstances what the magnitude of the biases may be. As Grayson suggests, the points he makes have been made previously by various authors and issues such as the power to detect dominance have been analyzed at some length. This paper draws attention to some other sources of variation which Grayson does not consider but which have been considered by other writers and which might have somewhat different consequences for the estimation of shared environmental effects. The classical twin study has never been an end in itself, but it is the nucleus of a systematic genetic approach to the study of human behavior.