Stand by me: Close friendships appear to counteract genetic vulnerability to depression in girls, but not boys

Publication of US psychiatry's updated diagnostic code has provoked renewed debate in recent weeks over the extent to which mental illness ought to be framed as a psychosocial or a biological problem. The answer of course is that it is both. A new Canadian study captures this interplay, showing how close friendships appear to mitigate the risk for girls whose genes mean they are more vulnerable than average to depression.

Mara Brendgen and her colleagues studied 294 pairs of twins aged ten years old (147 girls). Some of the twins were identical (they share the same genes), the others were non-identical (sharing just half their genes). Each twin pair was raised together in the same family.

The researchers obtained ratings of the children's signs of depression from their teachers and classmates. They also gauged their close friendships by asking each child to nominate up to three best friends in their class, and to indicate who their very best friend was. Reciprocal nominations were a sign of mutual friendship. The children also answered questions about the quality of their friendships, including whether they do fun things together or get angry with each other.

Consistent with past research, there was evidence of the role of genes in depression. That is, correlations in signs of depression were much higher between identical versus non-identical twins. If one of a pair of identical twins had signs of depression, this was taken as an indication that the second twin had genetic vulnerability for the condition. If one of a pair of non-identical twins showed signs of depression, this was also taken to mean the other twin had genetic vulnerability, but less so than in the case of identical twins.

Here's the main result. Genetic vulnerability to depression in girls was less likely to manifest if they had at least one close friend. Stated differently, the apparent protective effect of having at least one close friend was magnified in girls who were genetically vulnerable to the condition. This means that for girls there was an interplay between genetic risk and the protective effect of friendship. This was not the case for boys. Friendships did appear to protect boys from depression, but this was not related in any way to their genetic vulnerability. Perhaps, the researchers surmised, there is a gender difference because "girls tend to rely more on social relationships as a source of self-definition and self-validation, and their friendships are also characterised by greater intimacy, self-disclosure, empathy and emotional support."

Separate from any issues of genetic vulnerability, another gender difference was that boys, but not girls, showed an apparently additive protective effect against depression of having more friends. The researchers said this may be because girls more often have intimate one-on-one friendships, whereas boys are more often part of friendship groups.

Other details to emerge from the study: better quality friendships were more protective against depression (regardless of genetic vulnerability); genetic vulnerability to depression wasn't associated with the likelihood of a child having friends, but it was negatively associated with the perceived quality of their friendships.

The study has some limitations, particularly the relatively small sample size, the reliance on observer ratings of depression, and the cross-sectional design, which means a causal role for friendships cannot be assumed. It's possible that the manifestation of depression symptoms in genetically vulnerable girls leads to fewer friends, rather than more friends reducing signs of depression (note however that social support is a known mitigating factor against depression). Also, the results may be specific to this age group.

Despite these shortcomings, this is an innovative study on an important topic. Children who show signs of depression pre-adolescence are at heightened risk for having problems in their teens and beyond, so the more we understand about mitigating this risk, the better. The researchers said their results "emphasise the importance of teaching social interactional skills that promote positive relations with others to help prevent the development of depressive behaviour in children."

Further reading

Brendgen, M., Vitaro, F., Bukowski, W., Dionne, G., Tremblay, R., and Boivin, M. (2013). Can friends protect genetically vulnerable children from depression? Development and Psychopathology, 25 (02), 277-289 DOI: 10.1017/S0954579412001058