Nature or Drug Abuse? There are significant structural changes in striatal and prefrontal brain regions of stimulant drugdependent individuals. However, it is not clear if these brain abnormalities predate drug-taking, rendering individuals vulnerable for the development of dependence, or if these changes are the effect of many years of drug use. Ersche et al. (p. 601 ; see the Perpective by Volkow and Baler ) investigated brain abnormalities in both drug-dependent individuals and in their biological siblings who have never taken drugs of abuse and compared them with matched healthy volunteers. The brain abnormalities in the sibling pairs were associated with significant impairments in the regulation of behavior; an ability known to be compromised in drug dependence. Because these neural changes were observed in family members who do not take drugs, the changes are likely to represent neurological markers of vulnerability to addiction rather than consequences of chronic drug abuse.