Much effort in behavioural genetics has been motivated by the hope that one day human behaviour will be explicable in genetic terms. This hope is based on the assumption that the contribution of genetic and environmental factors to behaviour can somehow be partitioned. However, recent advances in genomics, cellular and developmental biology, and developmental psychology have made it clear that such partitioning is biologically implausible. The functional significance of genes or any other influence on phenotypic development can be understood only in relation to the organism–environment system of which they are a part. Genes and environments are neither alternative nor independent causes for phenotypic traits.

Key Concepts:

• Gene activity is involved in the production of all phenotypic traits.
  
• Partitioning behaviours into those specified by the genes and those that are the result of the environment is not possible.
  
• The causes of behaviour are always relational and distributed across internal and external factors and events.
  
• The influence of any single factor on behaviour (genes, hormones, diet, social interaction) is contingent on the organization of the entire organism–environment system.
  
• Genes do not have a privileged position in the development of behaviour, because they are themselves regulated participants in the developmental process.
  
• Modern developmental theory rejects attempts to separate the organism and environment.