How Family Size Affects Your Cognitive Performance

Can your siblings affect your cognitive performance? Multiple studies have found that many aspects of cognition—from education level to IQ—can be predicted by your birth order and your number of siblings. As part of Lumosity’s Human Cognition Project (HCP), we decided to see whether our own data on brain performance reflected these trends.

Over 12,000 members told us, in a recent HCP survey, how many siblings they had and what their birth order was (only child, oldest, middle, or youngest). We then analyzed BPI scores and demographic information from 10,207 of these members to find some fascinating results.

Sibling effects are not so simple!

Sibling number significantly predicted a person’s initial BPI (after controlling for birth order and other demographic factors)—but birth order itself was not predictive of BPI.

Specifically, we found an inverted U-shape relationship between sibling number and BPI. BPI peaked among members with 1-2 siblings and tapered off for members who had fewer or more siblings. This inverted U-shape was confirmed statistically by a significant negative quadratic effect of the number of siblings on initial BPI.

Our results mirror other researchers’ findings

A recent Personality and Social Psychology Bulletin study also found that sibling number predicted intelligence in children, but that birth order did not. Though similar to our findings, these researchers saw a negative linear trend: the more siblings, the lower a child’s IQ.

There are a number of possible explanations: we used adults rather than children; we have an international subscriber base; and finally, controlling for education might mask the relationship between education size and family level.

We were curious about the true relationship between education and sibling number, so plotted education length against sibling number (by estimating education length based on peoples’ reported education levels). We controlled for age in this analysis, since younger people may still be in school and thus be less educated.

Now, we know that education level is a predictor of baseline performance on Lumosity, so it shouldn’t be too surprising that this trend should also affect cognitive performance. To test whether this is true, we can look again at how family size predicts cognitive performance, but this time only control for the effects of age and gender.

Though the differences between this graph and the first graph are also small, now we see the highest peak at 1 sibling (though the differences between this are very small). This seems to indicate that the lower average education level of users with more siblings has an additional small effect on their cognitive performance.

What does this all mean?

While intellectual performance and sibling number are linked, researchers still hotly debate whether sibling number actually causes differences in intellect.

One theory is that the more siblings in a family, the thinner the parents must stretch their limited financial, emotional, and cognitive resources. Still another theory argues that the relationship between sibling number and intellect is really just a side effect—the real difference lies in the IQs of parents who choose to have smaller or larger families.

In either case, the fact remains that sibling number has quite a small effect in our database. The difference between users with two siblings and those with four or more was only about two points on an IQ scale—or a measly four percentile points. There was also huge variance, meaning that plenty of people with fewer siblings performed worse, and plenty of people with more siblings did better!
In other words, sibling number doesn’t dictate your brain’s performance—if you’ve ever looked at your BPI, you know that you can easily improve by 4 percentile points with just a few Lumosity training sessions!

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