For a yet-to-be peer-reviewed study published as a preprint on PsyArXiv, Ryan Charles Meldrum of Florida International University and two colleagues looked at data collected by the National Institute of Child Health and Human Development's Study of Early Child Care and Youth Development (SECCYD), a study performed in 10 US cities that followed thousands of families from their child's birth until they were 15 years old. Specifically, Meldrum and his team gathered intelligence test results of 715 "target" children at two different points in their childhood: one from age 10, another from age 15.

When the kids in the SECCYD were 11, researchers invited each child's best friend to complete a series of tests, including an IQ test. When Meldrum's team crunched the numbers, they found that the IQ of the target children was correlated with the IQ of their best friend at age 11. That is, they were statistically related — the way that, say, your weight is correlated with the amount of food you eat or your grades are correlated with how much you study. There are other factors at play, but you can at least say there's a relationship between the two.

There are all sorts of reasons kids might have similar levels of intelligence to their friends: birds of a feather flock together, after all. Except the team controlled for that: they factored out the target child's intelligence at age 10, their mother's IQ and education level, enrichment opportunities at home, family structure, and a handful of other variables that might throw a wrench into the results. Even after all that, the correlation stood.

It should be said that the intelligence tests used in the SECCYD were mostly a measure of "crystallized" intelligence as opposed to "fluid" intelligence — book smarts instead of street smarts, or what you know instead of how well you think. That makes sense, according to the researchers. "Individuals with more intelligent friends could be exposed to more enriching environments for increasing their knowledge, skills, and experience ..." they write. Plus, they continue, kids might study more and behave better in class if their smart friends are doing it too.

So thank your childhood BFF for your grown-up smarts! They have you to thank, too.

For a different take on how we measure intelligence, check out "IQ: A Smart History of a Failed Idea" by Stephen Murdoch. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.