X chromosome abnormalities and cognitive development: implications for understanding normal human development.

Recent advances in the biological sciences have offered new opportunities to identify biological contributions as they interact with social experience to help determine psychological development. The role of biological factors is more easily demonstrated in subhuman species in which extensive experimental manipulations of variables are possible. One strategy for the study of human behaviour genetics has been the systematic analysis of behaviour in individuals with naturally occurring X chromosome variations. The aim has been to demonstrate whether or not the range of expected variability in particular areas of behavioural development was narrowed by the specific genotypic abnormality. The knowledge obtained from these studies can be applied meaningfully to enhance our understanding about human behavioural development in chromosomally unaffected individuals.