Most psychiatric disorders show a tendency to run in families. This observation could be explained by genetic factors (nature) and/or by the influence of family and environment (nurture). Two long-established clinical research techniques have been used to distinguish these:

- Comparison of monozygotic with dizygotic twins. Any difference in the rates of illness between the types of twin can be attributed to the differences in the degree of sharing of genetic material between them.

- Comparison of monozygotic twins brought up together with those brought up apart. Here the only difference is in the environment, so any differences in rates of illness between the two groups can be ascribed to the environment.

• Adoption studies. These examine rates of psychiatric disorder in children whose biological parents were affected but who were brought up by healthy adoptive parents, or vice versa.

Twin and adoption studies, many of which have been carried out in Scandinavia where comprehensive national records of individuals and their health are maintained, confirm a genetic predisposition for the majority of psychiatric conditions. The effect is strongest for conditions such as autism and attention deficit hyperactivity disorder (ADHD), in which there is a roughly 50-fold increase in risk in relatives; in schizophrenia, the increase is 10-fold; and in anxiety and unipolar depression, it is up to fivefold (for review, see Tandon and McGuffin, 2002).

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