Peer deviance (PD) is associated with risk for drug abuse (DA). Is this association causal?

DA was recorded in official records. PD was defined as the percentage of peers residing in small communities with future DA registrations. We examined offspring in families whose community PD changed when the offspring was 0–15 years of age and then examined families where cousins or siblings differed in their years of exposure to low or high PD communities.

The duration of exposure to PD was strongly associated with future DA. Co-relative analyses for families whose exposure to PD declined suggested that the PD–DA association was largely non-causal. Within full-sibling pairs in such families, the length of exposure to low PD environments was unrelated to risk for DA. By contrast, co-relative analyses in families where exposure to PD increased over time indicated that the PD–DA association was largely causal. In such families, siblings who differed in the duration of their exposure to high PD differed in their risk for subsequent DA. These results were replicated in families whose PD changed because they moved or because of changes in the community in which they resided.

Within families whose social environment is improving over time, the association between PD exposure and offspring DA outcomes is not causal but is due to familial confounding. Within families whose social environment is deteriorating, the PD–DA association seems to be largely causal. Our measure of PD may also reflect broader aspects of the community environment beyond peers.

Using three independent methods, prior studies in Swedish sibling pairs indicate that environmental factors contribute substantially to familial aggregation for drug abuse (DA). Could we replicate these results in cousin pairs?

Using multiple Swedish public databases (1964–2011), we defined DA using medical, legal or pharmacy registry records and examined concordance in full cousin pairs as a function of age differences, younger–older relationships and geographical proximity while growing up.

Replicating prior results in siblings, cousin pairs were significantly more similar in their history of DA if they were (i) closer versus more distant in age and (ii) grew up in high versus low geographical proximity to one another. Furthermore, controlling for background factors, having an older cousin with DA conveys a greater risk for DA than having a younger drug-abusing cousin. The greater transmission of DA from older to younger versus younger to older cousin was more prominent in pairs who grew up close to one another. In age difference and geographical proximity analyses, effects were consistently strongest in male–male cousin pairs. In analyses of older → younger versus younger → older transmission, effects were stronger in male–male and male–female than in female–female or female–male relative pairs.

In accord with prior results in siblings, environmental factors contribute substantially to the familial aggregation of DA in cousins and these effects are, in general, stronger in males than in females.