Because environmental (both social and biological) and genetic factors are inextricably intertwined in families, it has been difficult to discern the relative influence of these possible causative factors in many child psychiatric disorders. Twin and adoption studies can be used to separate the effects of genetics from the social environment of rearing, and twin studies can often give clues to the importance of biological environmental factors. Both twin and adoption studies have been used to better understand the relative importance of these several causative factors how they interact. Genetic factors play some role in most psychiatric disorders in which symptoms begin during childhood or adolescence, but the severity of symptoms may also involve some type of environmental influence.