Recovering components of variance from differential ratings of behavior and environment in pairs of relatives

Evidence from genetic studies underscores the importance of examining the within-family environment and its association with genetic differences among siblings in influencing children's development. Simulated and published data are analyzed to illustrate the contribution of genetic and environmental factors to differential ratings of behavioral and environmental differences within families. Comparison of the mean differential ratings within various types of pairs is not an adequate index of the underlying genetic and environmental variance components. However, on the assumption that differential ratings are a linear function of underlying trait differences, it is a simple matter to recover within-family statistics that offer a more legitimate basis for estimating genetic and environmental components of variance within families. The assumptions and dangers underlying any analysis of differential ratings are noted.