[Twin studies in psychiatry].

Twin research in medicine and psychology has blossomed during the last decades. A brief presentation of twin research methods is given, followed by a review of psychiatric studies, particularly Norwegian ones. The Nordic investigations are of great importance because many of these studies have been able to avoid shortcomings in sampling by linking national twin registers to disease registers. This procedure is of consequence for the estimation of heritability. Twin research shows that the genetic contribution to Alzheimer's disease, manic-depressive (bipolar) disorder, schizophrenia and childhood autism is relatively strong, whereas genetic factors play a minor role in common depressions, anxiety and somatoform disorders, and alcohol abuse. Since the concordance rates are far from 100 percent in most of these illnesses, environmental factors must also be important.