Socioeconomic status and social support following illicit drug use: causal pathways or common liability?

Abstract

The negative social attributes associated with drug use and abuse/dependence may arise as a result of shared genetic or environmental factors rather than through causal pathways. To evaluate this possibility, structured interviews were conducted for 3969 male and female twins from the Mid-Atlantic Twin Registry and evaluations of their socioeconomic status (SES), social interactions, and use of drugs were obtained. Drug involvement was categorized as never used, tried, or met criteria for abuse or dependence. A co-twin control design was implemented using hierarchical linear modeling to assess whether twins who used drugs experienced lower SES and social support than non-using co-twins. Poorer social functioning in the drug-exposed twin is consistent with a causal relationship, while similar functioning in the drug exposed versus naive twins imply shared genetic or common environmental factors. Use of drugs was not significantly related to any SES measures. However, education and job status appear to share genetic influences with drug abuse/dependence. Lower income was not related to abuse/dependence of drugs. Negative interactions with friends and relatives share genetic factors with use of drugs, but the escalation from trying drugs to abusing them appears to generate discord between the abuser and friends and relatives in a causal fashion. These results indicate that presumptive causal influences of drug abuse/dependence on low SES may actually be mediated by shared genes. Drug use and social discord also appear to have shared genetic factors, but increased levels of drug involvement seem to causally influence social interactions.

Figure 1

Possible pathways between drug use or abuse and negative social outcomes. Note: A. Drug involvement may causally influence negative social outcomes B. Drug use or AD and negative social outcomes may have shared risk factors (genetic or environmental) in common.

Figure 2

The co-twin control method: theoretical and practical examples. Top, regression coefficient patterns expected for the general population, discordant dizygotic (DZ) twins, and discordant monozygotic (MZ) twins under three possible models. Bottom left, observed regression coefficients for the abuse of any drug and educational attainment demonstrating a noncausal genetic pattern. Bottom right, abuse of any drug and negativity from friends regression coefficients showing a causal pattern. Note: Pop = population average effect (family mean), DZ = dizygotic twins, MZ = monozygotic twins. Asterisks denote significant differences from zero at a p ≤ .05 level.