Dr Ivey Defoe Hello, everyone, I am Ivy Defoe, a Developmental Scientist working at the Child Development and Education Department of the University of Amsterdam, where I lecture and conduct research. During this video extract, I would like to share with you some of the main findings of one of my research publications that I led, and it also included two co-authors, namely Sanne Treffers, and Geert Jan Stams. The article is entitled “Siblings Matter a Multi-level Meta-analysis on the Association Between Cannabis Use Among Adolescent Siblings.” Okay, let’s jump right into the research question [pause]. Generally, for this study, we were interested in investigating whether cannabis use in sibling A predicts an increased likelihood of cannabis use, or cannabis use disorder, in sibling B. And this hypothesis was inspired by the Social Learning Theory, as well as the Developmental Neuro-Ecological Risk-taking Model theoretical framework that I have developed in a previous publication, back in 2021, in the journal Developmental Review. So, if you’re interested, you could have a look at it in that journal. Furthermore, we were also interested in the magnitude of this association, so was it small, medium or large? Additionally, we investigated factors that can possibly moderate this association. The factors we were interested in were the age of the youth, the birth order between the siblings, the age spacing between the siblings, and the gender of the youth in the study, as well as their sibling’s gender, and sibling type. So, think of identical twins, versus fraternal twins, versus non-twins. Moreover, when possible, we also conducted meta-analyses that investigated the association between peer youth cannabis use, so whether having a peer increases the likelihood that other youth within that peer network will also use cannabis. And additionally, we investigated the association between parent-youth cannabis use, as to whether having a parent who used cannabis – uses cannabis also increases the likelihood that their youth offspring will use cannabis. We had two primarily inclusion criteria, namely, we included studies that at least investigated the association between cannabis use among siblings. Furthermore, we were only interested in studies that included youth participants. We defined ‘youth’ as “the ages between 11 to 24.” Based on this inclusion criteria, we did a literature search. The results of the literature search are as follows. We found 20 studies on the association between sibling youth cannabis use. Now turning to the results. Those 20 studies resulted in 126 effect sizes on the association between sibling cannabis use. For this association, we found a significant large overall effect size. This means that having a sibling who uses cannabis increases the likelihood that the other sibling within that dyad will also use cannabis and that this association is large. Important to note here is that we found a stronger association among identical twins and same gender siblings. However, the association still existed significantly for other sibling types, such as, fraternal twins, non-twins. It was also significant across all gender types within the sibling dyad, and across all age groups. Next, we did a meta-analysis on the association between parent and youth cannabis use. We found a medium effect size, namely this indicates that when a parent uses cannabis, it’s an increased likelihood that the youth of this parent, so the youth offspring, will also engage in cannabis use. The effect size was a correlation of .30. Furthermore and lastly, we also looked at the studies that investigated whether there’s a relation between peer cannabis use and youth cannabis use. For those studies, we found a large overall effect size. Namely, having a peer who used cannabis predicts that other youth within that peer network will also use cannabis, and the correlation was .51 [means .451], which is a large correlation. The conclusion, generally, we could conclude that when it comes to sibling influences, this study showed that they are similar to the influences of peers when it comes to the strength of the magnitude. However, we found a stronger effect size for siblings versus parents. These results support the previous theoretical framework, DNERM, that I mentioned earlier, and they could suggest that the awareness or the observation of a sibling using cannabis could predict that the other sibling within that dyad might begin to normalise that behaviour, or it could spark their curiosity, which could eventually lead to increased levels of siblings cannabis use. Limitations, one limitation that I would like to emphasise, especially since I’m an advocate for more, let’s say, geographical diversity in scientific publications, a limitation related to that is unfortunately, we were only able to retrieve one study that was based on a non-Western sample, or a non-Western country. This unfortunately, means that strictly speaking, we cannot generalise these findings to non-Western countries. As you could imagine, in non-Western countries, the family system, so sibling dynamics, might be quite different compared to Western countries. For non-Western countries you could think of countries in Africa, some countries in Asia, as well as the Caribbean region, which I can also give evidence for, sibling dynamics there are quite different compared to siblings dynamics in the Netherlands, for example, which is a Western country. So, we need more research on siblings, and particularly research on siblings in non-Western countries, to be able to draw more, let’s say, firmer conclusions about the extent of sibling influences, especially compared to parents and peers. Alright, what’s the take-home message? There are some practical implications, I think especially when it comes to therapy for substance use disorders in youth. Since we found that sibling cannabis use predicts youth cannabis use to a large extent, it’s also important to consider siblings in therapy sessions for youth with substance use problems. These therapy sessions typically only include the parents of the youth, so perhaps including the siblings as well could increase the effectiveness of the therapies. Alright, those were the findings in a nutshell and their implications. I would like to thank you very much for viewing this video abstract. If you have any questions about the study, please feel free to contact me. Here is my email address, it’s i.n.defoe@uva.nl, and you could also consider finding me on LinkedIn, where I often share updates on my research publications. I’ve also listed my website here, for those of you who are interested in learning even more.