Behavioral development during childhood is examined in relation to recent concepts and data from evolutionary theory and developmental genetics. The epigenetic framework of Waddington is proposed as a powerful tool for analyzing the progressions in behavior, particularly for recognizing that development involves coordinated pathways of change over time. Many of these pathways appear to depend upon the activity of timed gene-action systems that switch off and on according to a predetermined plan. Behavioral development thus gives expression to the dynamics of preprogrammed change; and in this perspective, behavioral discontinuities may be as strongly rooted in the epigenetic ground plan as the continuities are. The present paper aims to pull together some common themes from different areas that bear on the central issues of behavioral development—the neural foundations, the time course followed, the interplay of maturation and experience, the extent of preorganization furnished by the genetic program, and the adaptive significance of such behaviors in an evolutionary perspective. The final section touches on some hypotheses drawn from developmental neurobiology and developmental genetics that may enrich the analyses of human behavioral development.

A new approach is proposed in twin research based on the study of twins who, though reared together, have subsequently lived apart for a period of at least five years. With respect to the more powerful study of twins reared apart, the twins living apart test has the advantages of being more realistic and affording easier access to sufficiently large samples. In this pilot study, the test has been applied to a sample of 92 monozygotic pairs now aged 35–45; 15 pairs were still living together and 77 had lived apart for over five years. As a first approach, comparisons have been made, in the cotwins of the two subsamples, with respect to the following traits: height, weight, presbyopia, presbyacusia, alcohol consumption, tobacco consumption, blood pressure (systolic and diastolic), dental caries, and hours of sleep.

The genetic and environmental effects on the levels of 17 serum biochemical quantitative traits (calcium, phosphorus, glucose, urea nitrogen, uric acid, total protein, albumin, total bilirubin, alkaline phosphatase, lactate dehydrogenase (LDH), glutamic-oxaloacetic transaminase (SGOT), total lipid, cholesterol, triglyceride, α-lipoprotein, pre-β-lipoprotein and β-lipoprotein) were estimated in 105 pairs of healthy twins of both sexes (57 MZ and 48 DZ) by path analysis. The genotype effect was significant for all traits (P < 0.001) and its value extended from 0.52 (α-lipoprotein) to 0.81 (alkaline phosphatase), whereas environmental effect was significant (P < 0.05) in only 10 traits of the 17 analyzed, with the maximum value of 0.13 (cholesterol). Correlations between genotypes of paired traits were estimated and, of 136 values, 47 were significant at the 5% levels, thus indicating partial and common genetic mechanisms.

Data with regard to the proportion of congenital malformations found at birth are presented from a survey of 657 pairs of twins of known zygosity and placentation delivered in Aberdeen and Northeast Scotland between 1968 and 1979. There was an excess of malformed individuals from monozygotic (MZ) pairs, but this did not reach statistical significance. The possible effect of monochorionic placentation in the causation of malformations in MZ twins in general is discussed. It seems that this type of placentation may be of less causative importance than has been previously suggested.

In their general model of twin concordance Allen and Hrubec recently introduced the concept of secondary ascertainment rate to consider partial ascertainment of secondary cases. This concept is found to be of limited interest because only pairs where valid information can be obtained from both twins should be included in genetic analyses. It is furthermore shown that the validity of the proband method and the Allen/Hrubec model depends on the equivalence of ascertainment in twins from concordant and discordant pairs. However, if this condition is fulfilled, it has been shown how the ascertainment probability in cotwins of probands is related to the overall ascertainment probability in the total twin population.