A Contagion Model for Within-Family Transmission of Drug Abuse

Abstract

Objective: The purpose of this study was to determine whether, controlling for genetic effects, drug abuse was transmitted within families as predicted by a contagion model.

Methods: The authors examined 65,006 parent-offspring, sibling, and cousin pairs ascertained from Swedish population registries in which the primary case subject had a drug abuse registration. The rate of drug abuse registration among at-risk secondary case subjects ages 19-23 was studied. Utilizing matched control pairs, a difference-in-difference approach was used to infer causal effects.

Results: In offspring, risk for drug abuse registration in the 3 years after an index registration of a parent residing in the same household, neighborhood, or municipality increased 5.9%, 3.4%, and 1.8%, respectively. For siblings of sibling index case subjects, parallel results were 5.9%, 3.9%, and 1.2%. For cousins of cousin index case subjects, excess risk for those in the same neighborhood or municipality was 2.9% and 0.9%, respectively. In all sets of relatives, drug abuse transmission was strongest in male-male pairs and in pairs closest in age. In sibling pairs, stronger transmission was observed in older to younger siblings compared with younger to older siblings. Transmission was stronger within than across the two drug classes with sufficient data (opiates and cannabis).

Conclusions: These results suggest that drug abuse can be transmitted within families by an environmentally mediated temporally defined model of contagion. The most important methodological limitation is that drug abuse registration is an inaccurate measure of the onset of drug abuse. Indeed, as predicted, drug abuse risk increased among potential secondary case subjects in the year before drug abuse registration of the index case subject.

Keywords: Families; Psychoactive Substance Use Disorder.

FIGURE 1.

Increase in risk for drug abuse (DA) registration among offspring, siblings, and cousins of index case subjects 3 years before and after DA registration^{a a} The graphs show the index relative as a function of the proximity of the index case and secondary case (orange dotted lines), results in offspring or siblings of control subjects (green dotted lines), and results expected in the at-risk secondary cases if the parent did not have an index registration for DA (black dotted lines). Error bars indicate 95% confidence intervals. SAMS=Small Area Market Surveys.

FIGURE 2.

Increased risk for drug abuse (DA) registration in potential secondary cases 1–3 years after DA registration of the index relative as a function of the proximity of the primary and secondary case subjects^{a a} Asterisks above columns indicate the statistical significance of the analysis. SAMS=Small Area Market Surveys. Error bars indicate 95% confidence intervals. *p<0.05.***p<0.0001.

FIGURE 3.

Increased risk for drug abuse (DA) registration in potential secondary cases 1–3 years after DA registration of the index relative as a function of sex and age relationships of the primary and secondary case subjects^{a a} Asterisks above columns indicate the statistical significance of the analysis; asterisks adjacent to the name of the variable under examination indicate whether the results in the analyses are statistically heterogeneous. Error bars indicate 95% confidence intervals. *p<0.05.**p<0.001. ***p<0.0001.

FIGURE 4.

Increased risk for drug abuse (DA) registration among potential secondary cases 1–3 years after DA registration of the primary case subject for parents, siblings, and cousins as a function of the source of registration for DA among the primary and secondary case subjects^{a a} Cr=criminal registry, Med=medical registry. Asterisks above columns indicate the statistical significance of the analysis; asterisks adjacent to the name of the group of relatives indicate whether the results in the class are statistically heterogeneous. Error bars indicate 95% confidence intervals. *p<0.05.**p<0.001. ***p<0.0001.