By Christian Jarrett

Picture yourself aged 11: who was your best friend and how smart were they? The answer may have shaped your life more than you think. A new study published as a pre-print at PsyArXiv reports that participants’ IQ at age 15 was correlated with the IQ of whomever was their best friend years earlier, when that friend was aged 11, even after factoring out the participants’ own earlier intelligence, as well as a host of other potentially confounding variables.

We already know, thanks to previous research, that our school-age peers shape our personalities, our powers of self-control, and the chances that we’ll get into trouble, so it’s to be expected that they also affect our intelligence (and we theirs). Surprisingly, however, this possibility had not been studied before now. “Our findings add … another layer of evidence for the important and pervasive influence of peers on a host of traits during adolescence,” the researchers said.

Ryan Charles Meldrum at Florida International University and his colleagues used data collected from hundreds of families resident in 10 US cities between 1991 and 2007 as part of a study run by the National Institute of Child and Human Development. This included intelligence test results from 715 participating kids completed when they were aged 10 and then again when they were aged 15. These “target” kids’ individual best friends had also completed an intelligence test when they were aged 11-12 (most best friends were the same sex as the target kids and they were no more than two years older or younger).

The IQ of the target kids at age 15 was strongly correlated with the IQ of their best friend when their best friend was aged 11-12. This could be due largely to the fact that children like to be friends with other kids who are similar to them. Critically, however, the target kids’ IQ at age 15, based on a composite of three tests, was still associated (β = 0.08 in a multiple regression) with their best friends’ IQ at age 11-12 even after factoring out their own IQ score when they were aged 10-11, as well as a range of more than nine other confounding variables, such as their mothers’ IQ and education level, and the “enrichment” opportunities in their home. This provides tentative evidence that the target kids hadn’t just chosen childhood friends with an IQ similar to their own, but that their IQ in mid-adolescence had actually been shaped by the intellect of their best friend years earlier.

The new study can’t speak to how a childhood best friend affects our future intelligence, but one can imagine that if your best friend were motivated to study hard, this will have boosted your own academic motivation, with lasting positive consequences for your IQ. Just by hanging out with a brainy buddy, you will also likely have absorbed some of their knowledge and skills. Conversely, if your best friend was not such an intellectual, you will not have enjoyed these cerebral benefits.

One caveat to bear in mind is that the IQ tests used in this study, such as the Peabody Picture Vocabulary Test, tap what’s known as “crystallised intelligence”, which refers to our knowledge, rather than “fluid intelligence”, which is more about mental agility. There was also no measure of fathers’ IQ which may have confounded the results, potentially influencing the IQ of the target kids and who they chose to have as a best friend. Future research also needs to look at the effect of peer group IQ in aggregate, not just an individual best friend, although presumably their influence will be the strongest.

While acknowledging these and other limitations of their research, Meldrum and his colleagues said “overall, our results provide support for the hypothesis that having more intelligent friends is associated with higher future levels of intelligence.”

— On the Longitudinal Association between Peer and Adolescent Intelligence: Can Our Friends Make Us Smarter? [This article is a pre-print and has not yet been subjected to peer-review]

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest