Explain the interactionist approach to explaining mental disorders.

The biological approach to explaining mental disorders would suggest that arise as a result of genetic vulnerability combined with an environmental 'stressor' event which triggers the expression of such genes. Evidence supporting this idea is derived from the fact that in monozygotic (identical) twins, concordance rates (where one twin has OCD, the occurrence rate of the other twin also having OCD) consistently tend to fall around 50% in most studies of this. As monozygotic twins share 100% identical DNA, the fact that concordance rates of OCD are only 50% suggests that OCD is not entirely determined by genetics, and also partially influenced by environmental influences.