The results of this study indicate that overall, both parental alcohol and drug dependence are associated with similar and elevated levels of risk for a broad range of externalizing disorders among offspring. This extends research on the generalized transmission of risk for externalizing psychopathology by demonstrating that two specific parental externalizing disorders – alcohol and drug dependence – both relate to risk for a broad range of childhood and adolescent disruptive behavior, antisocial, and substance use disorders. This is consistent with the idea that an externalizing disorder – specifically alcohol or illicit drug dependence – in parents confers risk for offspring externalizing disorders in part through a general genetically influenced vulnerability toward externalizing disorders that is transmitted from parent to child (Hicks et al. 2004
As highlights, compared to families where neither parent is dependent on alcohol or illicit drugs, the offspring of parents with alcohol, cannabis, or other illicit drug dependence are at substantially elevated risk for all of the externalizing outcomes (e.g. prevalence rates were at least three times higher in these families for all adolescent outcomes). However, after adjusting for the effects of parental alcohol dependence, the effects of parental cannabis dependence are somewhat weaker than the effects of other drugs, with the effects of cannabis dependence significant only for alcohol and drug dependence among offspring. There are several possible reasons for this. Cannabis is often viewed as a ‘gateway drug’ – i.e. not as severe as the use of other drugs, and a step on the pathway toward other drug use (e.g. Kandel, 2002
). It seems possible, then, that individuals who become dependent on cannabis but do not become dependent on other ‘harder’ drugs may have influences that contribute to adaptive functioning (e.g. a good family life, a steady job). In addition, cannabis use is more common than the use of other drugs and therefore heavy use may be associated with less deviance (e.g. people who use cannabis may have more ability to maintain work and relationships than people who use other drugs). Cannabis may also be easier to access than many other drugs, making association with more deviant individuals less typical of cannabis users compared to users of other drugs.
Rates of psychopathology among offspring of substance-dependent parents in this study were generally consistent with the range of rates reported in previous studies, although our rates of CD were somewhat higher than previous estimates. This study also adds to the descriptive literature by reporting rates of ODD, AAB, and nicotine dependence among these offspring and also by separating out the rates of externalizing psychopathology among offspring of cannabis-dependent and other drug-dependent parents. As such, it builds on existing research by further broadening our understanding of the degree to which parental substance dependence is associated with a wide range of externalizing outcomes.
This study has clinical implications. It is clear that child and adolescent offspring of parents with histories of substance dependence of any sort are at risk for a wide variety of negative outcomes. Although parental cannabis dependence is associated with weaker risk to offspring than parental dependence on other drugs, the relatively weaker effect of cannabis dependence is unlikely to warrant different assessment or treatment strategies. It would be prudent to inquire about a history of substance dependence among parents of youth presenting for psychological evaluation or treatment of externalizing disorders; in addition, inquiry into the type of substance dependence would be appropriate. It would also make sense to evaluate and implement preventative or treatment interventions for children of adults who enter substance abuse treatment programs. In addition, it is worth noting that among the offspring disorders examined in this study, substance dependence is typically one of the last to develop [i.e. although sequence effects were not examined, ADHD must be present prior to age 7 to be diagnosed, CD must be present prior to age 15 to be diagnosed, and ODD typically has an onset prior to age 8 (APA, 1994)]. Therefore, offspring of substance-dependent parents who are exhibiting one of these other disorders earlier in childhood warrant particular attention because they may be especially prone to develop later substance dependence.
This study has some limitations. Participants in this study were primarily Caucasian; although they were representative of the population of families with at least one child in the state of Minnesota at the time the adolescent participants were born (Holdcraft & Iacono, 2004
), it is not known how well these findings would generalize to other groups. In addition, different results could be expected for offspring in studies of parents with severe or persistent alcohol and/or drug addictions. Moreover, although the uniform age of the offspring was a strength in that we were able to examine these issues among adolescents on the cusp of a key developmental transition (i.e. high-school graduation) and specifically focus on early-onset cases of substance dependence, we cannot be certain that these findings would be the same for younger offspring whose risk is less likely to be expressed or in older offspring who have settled into adult patterns of drug and alcohol use and whose externalizing behavior may have persisted into adulthood or desisted. In addition, it is striking that we found these effects using lifetime diagnoses, since parents may have had alcohol and/or drug dependence before the offspring were born. Future research examining how the timing of parental substance use disorder relates to offspring outcome would be useful. Further, the slightly differing patterns of results between cannabis and other drug dependence could be due to a multitude of other factors that may differentiate families in which the parents have a history of cannabis dependence compared to those in which the parents have a history of dependence on other drugs. A comprehensive examination of these factors was beyond the scope of this paper, but an examination of possible differences would be helpful in explaining the present results.
This study represents an examination of externalizing disorders among late-adolescent offspring of alcohol- and drug-dependent parents in a community-based sample. The results of this study support the notion that overall, parental alcohol and drug dependence are both associated with broad-based risk for a range of externalizing psychopathology to offspring. Future research investigating the mechanisms driving these associations would be useful and possibly helpful for developing targeted preventative interventions for externalizing disorders among offspring of parents who have histories of dependence on any substance.