Kaja LeWinn, from the University of California, San Francisco, demonstrated this by reanalyzing data from a large study that scanned 1,162 children ages 3 to 18 to see how their brain changed as they grew up. The kids came from disproportionately wealthy and well-educated families, so LeWinn adjusted the data to see what it would look like if they had been more representative of the U.S. population. That's called “weighting,” and it’s a common strategy that epidemiologists use to deal with skews in their samples. As an easy example, if you ended up recruiting twice as many boys as girls, you’d assign the girls twice as much “weight” as the boys.

When LeWinn weighted her data for factors such as sex, ethnicity, and wealth, the results looked very different from the original set. The brain as a whole developed faster than previously thought, and some parts matured earlier relative to others.

Natalie Brito, from New York University, says that this study “clearly shows how our interpretation of brain development changes based off who is being represented within the sample.” She adds that most neuroscientists would acknowledge or agree that representative samples are a good thing, but that there are practical reasons why such samples are hard to get. Most obviously, brain-scanning studies are very expensive, so most of them are small and rely on “samples of convenience”—that is, whoever’s easiest to recruit.

“Neuroimaging research is also complex and difficult to conduct, and because of this, I think there is a tendency to focus on its technological aspects,” says Duke Han, from the University of Southern California. That’s symptomatic of a problem in neuroscience that I’ve written about before: an inclination to focus on the technological innovations that allow us to study the brain, and to forget about the people whose brains are being studied.

Perhaps the brain itself invites this lapse. We intuitively understand that our thoughts and behavior vary considerably from person to person. But when it comes to the lump of gray tissue behind those behaviors, it’s easy to forget that variation. “To a degree, I think there’s a sense that a brain is a brain is a brain,” says LeWinn. “That’s problematic. Everyone’s brain is shaped by their experiences, and we want to capture the diversity of people’s experiences rather than just a few kinds.”

For example, in the study she reanalyzed, around 35 percent of the children had parents with college backgrounds, and around 38 percent had parents who earned more than $100,000 a year. If the sample had been truly representative of the U.S. population, those proportions would have been 11 percent and 26 percent, respectively. And weighting the data to account for these biases produced a different picture of brain development.

Brains get bigger as we get older, before shrinking again during later childhood. In the unweighted data, the brains hit their peak volume at 6 years on average, and their peak surface area at around 12 years. But in the weighted data, the brains hit those milestones 10 months and 29 months earlier, respectively. The pattern of development across the brain also changed. In the unweighted data, three of the brain’s four lobes hit their maximum area from the ages of 12 to 13, with only the parietal lobe peaking earlier at around 10 years. But the weighted data showed more of a wave of maturation, from the back of the brain to its front, and going from 9 years to 11.