Parental separation and early substance involvement: Results from children of alcoholic and cannabis dependent twins
Introduction
Compared with children of non-alcoholic parents, children of alcoholics (COAs) report earlier and more frequent use of alcohol as well as tobacco, cannabis, and other illicit drugs, and are at greater risk of alcohol problems during adolescence and adulthood (Lieb et al., 2002, Schuckit and Smith, 1996, Sher et al., 1991). By some reports, COAs are four to six times as likely to develop an alcohol use disorder at some point in their life (Chassin et al., 1991, Russell, 1990). Comparatively less is known about children whose parents abuse other drugs; however, paternal illicit drug use has been linked with earlier tobacco use, and problem use of alcohol, tobacco and illicit drugs (Clark et al., 1998, Clark et al., 1999).
While risks for early and problem substance use associated with parental alcoholism are widely documented, not all COAs initiate use at early ages, and for those offspring who show signs of problem use, many “mature out” during adulthood (Labouvie, 1996, Maisto et al., 2002). Furthermore, COAs experience a range of adversities that often follow from but are not exclusive to parental alcoholism, and many such “non-specific” risks have considerable consequences (Jacob and Johnson, 1997). Parental separation or divorce provides a strong example as alcoholic parents are at increased risk of marital dissolution (Waldron et al., 2013) and compared to children from intact married families, children of divorce also report earlier use of alcohol, tobacco, and cannabis (Hoffman and Su, 1998, Short, 1998), heavier use of these substances (Doherty and Needle, 1991, Hoffmann, 1995, Needle et al., 1990), and higher rates of problem use (Fergusson et al., 1994, Hoffmann and Johnson, 1998).
Surprisingly, risks to offspring associated with parental separation have received limited attention in research on children of alcoholic or other drug addicted parents. Using a Children-of-Twins (COT) design (Gottesman and Bertelsen, 1989, Heath et al., 1985, Nance and Corey, 1976), we examine whether parental separation predicts early substance involvement over and above risks from parental alcohol or cannabis dependence, including genetic risks. Genetic variation has been reported for alcohol abuse and dependence (Heath et al., 1997, McGue, 1994) and a variety of drug use disorders, including cannabis abuse and dependence (Kendler and Prescott, 1998, Lynskey et al., 2002), with genetic variation also observed for initiation, regular use, and problem substance use during adolescence (Maes et al., 1999, McGue et al., 2000, Rhee et al., 2003). Heritable influences on marital status are reported as well, including genetic variation in likelihood of marriage (Trumbetta et al., 2007) and risk of divorce (McGue and Lykken, 1992), with at least one report of genetic covariation between alcohol dependence and both marital timing and survival (Waldron et al., 2011).
In COT studies, genetic and environmental risks are inferred from parent and cotwin history of substance dependence, with outcomes of offspring from a minimum of four groups compared, each with varying degrees of genetic risk and environmental exposure. In the present analysis, these groups include: offspring whose parent is substance dependent (Group 1); offspring of an unaffected parent whose monozygotic (MZ) cotwin is substance dependent (Group 2); offspring of an unaffected parent whose dizygotic (DZ) cotwin is substance dependent (Group 3); and offspring from control families, where neither parent nor cotwin, regardless of zygosity, is substance dependent (Group 4). Hypothesized risks to offspring are summarized in Table 1.
Following from quantitative genetic theory, if the association between parental substance dependence and offspring substance involvement results from rearing environment, offspring of affected parents should demonstrate greater risk, compared with unaffected parents (Group 1 > Groups 2–4). If the association results from genes shared between parents and their children, i.e., genetic transmission, offspring at high genetic risk should exhibit earlier involvement than offspring at intermediate genetic risk regardless of environmental risk (Groups 1 and 2 > Group 3). A pattern consistent with gene–environment interaction (G × E) is evident if offspring reared by an alcoholic or drug dependent parent exhibit greater risk, compared to offspring of unaffected parents, with offspring of an unaffected parent whose cotwin is also unaffected at lowest risk (Group 1 > Groups 2–3 > Group 4).
To date, a handful of COT studies of alcoholic families have been conducted. For early and problem use of alcohol, evidence of environmental transmission from parental alcoholism has been documented in some but not all reports (Duncan et al., 2006, Jacob et al., 2003, Sartor et al., 2007, Slutske et al., 2008). COT studies based on twin and cotwin history of divorce have been conducted as well, with evidence broadly suggestive of environmental transmission across a range of substance use outcomes (D’Onofrio et al., 2005, D’Onofrio et al., 2007). The present study is distinct from earlier work in that we examine timing of alcohol, cigarette and cannabis involvement as a function of parental separation or divorce, employing a COT design to control for genetic and environmental risks from parental substance dependence, including risks from parental cannabis dependence.
Section snippets
Participants
Participants were drawn from two studies of Australian children of twins selected from a young adult twin panel born between 1964 and 1971 (Heath et al., 2001, Knopik et al., 2006). Following initial contact by mailed questionnaire in 1989 (thus, the “1989” cohort), twins completed diagnostic telephone interviews during 1997–2002. Pairs where at least one twin had biological children ages 7–24 and one twin met DSM-IV criteria for alcohol use disorder (AUD; operationalized as alcohol dependence
Results
There were no differences in offspring age or sex by parental AD or cannabis dependence. Offspring of separated parents were slightly older (r = 0.19, p < 0.0001), with no differences in offspring sex by parental separation. Twin parents with a history of AD or cannabis dependence were slightly younger at the 1997–2002 assessment (r = −0.06 and −0.07, p < 0.05, respectively), with fewer female twins than male meeting criteria for either disorder [OR = 0.54 (95% CI: 0.42–0.70) and OR = 0.55 (95% CI:
Discussion
Despite well-documented associations between problem substance use and relationship instability, risks to COAs associated with parental separation or divorce have received limited empirical attention, and this is especially true of children of parents with other drug dependence. In the present study, we examined initiation of substance involvement as a function of parental separation using a Children-of-Twins (COT) design to control for both genetic and environmental risks associated with
Role of funding source
Funding for this study was provided by NIAAA Grants AA011998, AA07535, AA07728, AA010249, AA015210, AA017688, and NIDA Grants DA023696 and DA14363; the NIH had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.
Contributors
Heath, Bucholz and Martin designed the study and Heath, Bucholz, Glowinski and Waldron wrote the protocol. Waldron managed the literature searches and summaries of previous related work. Waldron undertook the statistical analysis, and author Waldron wrote the first draft of the manuscript. All authors contributed to and have approved the final manuscript.
Conflict of interest
No conflict declared.
Acknowledgements
We thank Richard Parker and Monica DeNooyer for their assistance with data collection, and the twin families who participated in the studies.
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