Abstract
Can drug abuse (DA) be transmitted psychologically between adult siblings consistent with a social contagion model? We followed Swedish sibling pairs born in 1932-1990 until one of them, sibling1 (S1), had a first DA registration. We then examined, using Cox regression, the hazard rate for a first registration for DA in sibling2 (S2) within 3 years of a first DA registration in S1 as a function of their geographical proximity. We examined 153 294 informative pairs. To control for familial confounding, we repeated these analyses in sibships containing multiple pairs, comparing risk in different siblings with their proximity to S1. DA was recorded in medical, criminal or pharmacy registries. The best-fit model predicted risk for DA in S2 as a function of the log of kilometres between S1 and S2 with parameter estimates (±95% confidence intervals) of 0.94 (0.92; 0.95). Prediction of DA included effects of cohabitation and an interaction of proximity and time since S1 registration with stronger effects of proximity early in the follow-up period. Proximity effects were stronger for smaller S1-S2 age differences and for same- v. opposite-sex pairs. Sibship analyses confirmed sibling-pair results. Consistent with a social contagion model, the probability of transmission of a first registration for DA in sibling pairs is related to their geographical proximity and similarity in age and sex. Such effects for DA are time-dependent and include cohabitation effects. These results illustrate the complexity of the familial aggregation of DA and support efforts to reduce their contagious spread within families in adulthood.
Highlights
We have recently examined drug abuse (DA) among parent–offspring, sibling and cousin pairs in Sweden showing stronger transmission in those living together v. those residing in the same town which in turn was stronger than those living only in the same large metropolitan area (Kendler et al, 2019)
We examine whether the effect of proximity on risk for DA in S2 risk attenuates over time
The DA density in the small areas for market statistics (SAMS) region in which S2 was residing was higher when S2 was v. was not registered for DA (3.2 v. 1.8%)
Summary
This study utilised several Swedish population-based registers with national coverage, the availability and content of which have been described previously (Kendler et al, 2013a). From the Swedish Multigenerational register, we included in the study database all possible full-sibling pairs, where both individuals within the pair were born between 1932 and 1990, and had a maximum age difference of 10 years. For pairs born prior to 1960, we required that both individuals were alive at 1985, and for individuals born 1960 and onwards we required that both were alive at age 15. The restriction that both siblings should be alive at age 15 was set because this is the age when individuals can be registered for DA in the criminal registers. The restriction that individuals born prior to 1960 should be alive at age 25 is due to the fact that most of the registrations for DA occur before age 30 and the ascertainment for DA in the relevant registers are probably less complete during the 1970s and 1980s
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