Cannabis, the most widely used drug among youth, comes with significant risks according to numerous studies. Its use during adolescence has been associated with cognitive difficulties, breathing problems, hallucinations, paranoia, and increased likelihood of cannabis dependency in adulthood. ‘Perhaps unexpected to many, in countries such as the USA and the UK, cannabis is the substance for which youth most often seek treatment and it is the most diagnosed substance use disorder in the Netherlands’, explains Ivy Defoe, researcher Forensic Child and Youth Care Sciences at the University of Amsterdam and one of the authors. ‘We need to know more about the triggers of cannabis use to help prevent mental problems and addiction.’

What is already known is that social factors play an important role for cannabis use during the youth period. It is for example well documented that it matters if parents and peers use cannabis. But less is known about the influence of siblings. ‘And this is surprising’, says Defoe. ‘Youth typically spend more time with their siblings compared to their parents and friends or peers. And siblings are typically both family ánd peers, which we would expect to make their influence even larger.’

The relation between sibling-youth cannabis use and disorders

With colleagues Defoe pioneered an investigation into the relation between sibling-youth cannabis use and disorders. They also analysed in how far this relation was influenced by sibling type (twins versus non-twins), gender and age. ‘Through our meta-analysis of  20 studies we wanted to robustly establish to what extent siblings’ cannabis use influences youth cannabis use. When comparison data were available, we also compared these with the influence of parents and peers.’

Siblings’ influence, same as peers but stronger than parents

The findings of the studies revealed a strong connection between cannabis use by youth and their siblings. ‘When a sibling uses cannabis, it increased the likelihood of youth also using cannabis’, says Defoe. ‘This association was particularly notable among identical twins and same-gender siblings, but it was also found among fraternal twins, non-twins, across genders, and all age groups.’ Interestingly, they found the influence of siblings to be as strong as friends and peers, and even stronger than the influence of parental cannabis use.

It is not just genetics

So is it all genetics? ‘No’, state the authors, ‘if genetics were the only factor, parents would have just as much influence as siblings, and siblings would have more influence than friends and peers.’ Since this is not the case, the authors believe environmental influences, like social learning, play an important role and contribute to the link between siblings and youth cannabis use.

‘Research shows that youth typically spend more time with siblings and peers than with parents, making the observation of sibling cannabis use seem normal’, is how Defoe explains this environmental influence consistent with her Developmental Neuro-Ecological Risk-taking Model (DNERM). ‘This normalization of behavior can increase attraction and engagement in cannabis use. This is especially strong for adolescents who are still exploring their identity and developing self-control. Simply observing a sibling using cannabis can spark their curiosity and potentially lead to their own cannabis use.’

Siblings in therapy

Therapies for youth with cannabis use problems could greatly benefit from including siblings conclude the authors. ‘Siblings are more accessible than peers for such therapies. Unfortunately, most substance use therapies for youth tend to focus solely on the parent-child relationship, neglecting the important role of siblings. Our meta-analysis emphasizes that all types of sibling constellations are relevant when addressing youth cannabis use.’

Article details

Defoe, I.N., Treffers, S. and Stams, G.J. (2023), Research Review: Siblings matter. A multi-level meta-analysis on the association between cannabis use among adolescent siblings. J Child Psychol Psychiatr. https://doi.org/10.1111/jcpp.13836