An appropriate family study design can avoid such confounding effects (Eaves and Sullivan 2001). Equally important, studies involving multiple siblings from one family allow investigators to distinguish between direct effects of a risk factor and correlated effects of other risk factors related to the families’ background that may be more common in certain families. One example of such a situation is childhood sexual abuse, a childhood trauma with major effects on the risk of psychopathology. Thus, family studies can help determine if the risk of alcohol dependence is directly affected by childhood sexual abuse or by other family background factors that are more common in families where abuse occurs. In the former case, only the affected child would be at increased risk of alcohol dependence, whereas in the latter case all siblings, even those who had not been abused, would be at increased risk. Well-designed family studies can identify these differences, as well as the possible interactions of direct and indirect effects on risk with differences in genetic vulnerability.
