Twins and chronogenetics: correlated pathways of development.

Developmental processes in infant twins are considered from the standpoint of how closely synchronized the twins are for growth spurts and lags, and whether the course of mental development follows the same trajectory as physical growth. The episodes of acceleration and lag may be independent for the two domains, so than an infant who is precocious in physical growth may not necessarily be advanced in mental development. The phasing of maturational episodes appears to depend upon timed gene-action systems which unfold in sequential order, and which promote new behavioral capabilities as well as physical growth. Data are reported from a large sample of MZ twins that were followed from 3 months to 6 years of age, with measures made of height and mental development. The measures were analyzed separately for the degree of twin concordance over ages, as an indicator of how closely the twins' growth spurts were synchronized. Then the measures were correlated to determine whether the episodes of acceleration and lag occurred at the same time for height and mental development, or were out of phase. The results showed only a low-order correlation among measures, indicating that the episodes of rapid growth and lag were essentially independent for the two domains. By contrast, the twin concordance was very high for each measure separately. The results are interpreted in the broader context of timed gene-action systems of extraordinary precision which initiate and regulate the episodes of growth.