Bias in the design of studies which examine genetic and environmental influences on behaviour

Genetic and environmental factors may interact to influence the development of complex disorders such as schizophrenia, depression and alcoholism. Twin and adoption studies are used to examine the relative influence of these factors on such behaviours. We propose a set of guidelines for critiquing these studies. Results of the methodologically sound studies may be useful in identifying high risk populations and designing and implementing prevention and treatment programs.