Research Review: Siblings matter. A multi-level meta-analysis on the association between cannabis use among...

Parents' and peers' cannabis use are well-documented predictors of youth cannabis use, however, relatively little is known about the influence of siblings' cannabis use. Hence, this meta-analysis investigated the association between sibling-youth cannabis use (disorder) and explored moderation by sibling type (monozygotic- vs. dizygotic- vs. non-twins), age, age spacing, birth order, gender, and gender constellations (same- vs. mix- gender pairs). When comparison data of parents' and peers' cannabis use (disorder) were also available in the included studies, separate meta-analyses on associations between parent-youth and peer-youth cannabis use (disorder) were additionally conducted.

Studies were selected if they included 11- to 24-year-old participants, and investigated associations between cannabis use (disorder) among those youth and their siblings. These studies were retrieved via a search in seven databases (e.g., PsychINFO). A multi-level meta-analysis using a random effects model was performed on the studies, and heterogeneity analyses and moderator analyses were also conducted. PRISMA guidelines were followed.

We retrieved 20 studies (most of which originated from Western cultures) with 127 effect sizes for the main sibling-youth meta-analysis and found a large overall effect-size (r = .423), implying that youth had higher cannabis use rates when their sibling used cannabis, and this association was stronger for monozygotic twins and for same-gender sibling pairs. Finally, a medium effect size existed for the associations between parent-youth cannabis use (r = .300) and a large effect size for peer-youth cannabis use (r = .451).

Youth are more likely to use cannabis when their siblings use cannabis. This sibling-youth cannabis use association existed for all sibling constellations, was larger than the association between parent-youth cannabis use, and was similar in magnitude compared to the association between peer-youth cannabis use-suggesting both genetic and environmental influences (e.g., social-learning) between siblings. Hence, it is important not to neglect sibling influences when treating youth cannabis use (disorder).