How do friends affect our brain function?

It goes without saying that friends are very important to us, and we begin to establish friendships from a young age. This influence provides a massive stimulus for brain development. But how does it affect the brain, and what’s the ideal number of friends? As she explains to Chris Smith, by looking at data collected from thousands of children, correlating cognitive abilities with imaging results and the number of friends each has, Fudan University’s Chun Shen has found that up to 5 appears to be ideal. Beyond 5 friends, there are no more benefits to mental health or cognitive abilities. Instead, the reverse may be true: keeping up with too hectic a social life may prove destructive…

Chun - Children, in late childhood, the influence from peers and friends becomes more and more important for children's health and cognition. So we are very curious about whether having more and more friends is necessarily better for children. So there are two main aims, whether more close friends is better or not, and there's the potential mechanisms underlying these relationships.

Chris -  How did you explore both of those things?

Chun -  We think if we want some very solid conclusions, we need a big data set. So first we used the Adolescent Brain Cognitive Development study, which included 7500 children, nine to 10 years old. And in this dataset they have very rich behavioural measures and also bring major measures. So we can use this data set to investigate the relationship between close friendships and mental health and cognition. And we also can use this data to explore the relationship between close friendship and brain development.

Chris -  Naturally this is an observational study, so what you are able to say is we can look at how many friends a person has and we can look at the brain imaging and we can draw associations between the two. But I suppose what you can't say is whether one begets the other, it's chicken or egg. Did the brain changes make the social changes or did the changes socially have more or fewer friends then? Because the brain changes.

Chun -  Yes, yes. Essentially it is observational study, so can't make conclusions. I agree with you.

Chris -  Nevertheless. What were the relationships that emerged when you looked at this? It's a very big group of individuals that you've been able to look at. What were the relationships that you saw between brain changes and friendship and social circumstances?

Chun - We foound non-linear relationships between the number of close friends and mental health, especially social and attentional problems and cognitive performance intelligence.

Chris -  Is there a sweet spot then, a certain number of friends which, up to that point, you get good results from? And then beyond that things level off or get worse? How does the graph look if you drew a graph of benefits versus disbenefits and the number of friends? What does the graph look like?

Chun -  Our study, we found that the positive relationship between close friends and mental health diminished beyond five close friends. And for cognitive measures beyond five close friends, the relationship became negative, which means more close friends, worse cognitive performance.

Chris -  Do you know why the number five appears to be special here? It's interesting, isn't it: we've got five fingers, five toes on each limb. And also if you ask people to look at a picture and estimate how many things that there are in it, we're very good at guessing up to five and beyond that things get more tricky. Is there something special about the number five in the human brain then?

Chun -  We also think the number of five is very interesting. I think this makes sense because too few friends means that you have no one to interact with. If some of them are busy, too many friends probably means they are not very closely connected to you. There's maybe a trait of between the quantity and quality of friendship networks spending too much time on social activities may lower your academic performance.

Chris -  Do we know, if you look at the brain imaging, how these friendship groups and access to friends and social networks affects or appears to affect, notwithstanding the fact that we're only making an observational association here. But how does the number of friends correlate with the brain structure in these individuals?

Chun -  The relationship between close friendship, quantity and the brain structure is very similar to the nonlinear relationship between the number of close friends and mental health. We found a strong relationship between close friendship, quantity and social brain regions, which means that these brain regions are responsible for social processing. However, the positive relationship diminished when the number of close friends exceeds around five.