INTRODUCTION

Developmental behavior genetics is concerned with the diverse ways in which genetic and environmental processes are involved in changes as well as continuity in development (Plomin, 1986; DeFries & Fulker, 1986). During ontogenesis, observed (phenotypic) change of a quantitative character may be due to distinct subsets of genes turning on and off, whereas continuity, on the other hand, may be caused by stable environmental causes. In contrast to the popular point of view, then, genetically determined characters are not always stable, nor are longitudinally stable characters always due to hereditary influences. Only through carefully designed longitudinal investigation of phenotypic changes in genetically related individuals can the dynamic patterns of genetic and environmental influences be disentangled.

In the following we shall mainly be concerned with a particular type of genetic model for the analysis of longitudinal phenotypic data, namely the simplex model (Jöreskog, 1970). The genetic simplex model is a genuine time series model and therefore can explain the characteristic time-dependent patternings of serial correlation (autocorrelation) as observed in longitudinal studies. It was already shown by Cronbach (1967) that common factor analysis of autocorrelation matrices will yield spurious, i.e. invalid, results. Consequently, recent efforts in the genetic modeling of longitudinal data have put particular emphasis on the elaboration of simplex models in this context (Boomsma & Molenaar, 1987a; Eaves, Hewitt & Heath, 1988).