Aaron Wichman, a psychologist at Ohio State University, and colleagues at the University of Oklahoma and University of North Carolina studied 3,000 families as part of a national study from 1986 to 1998, the Columbus Dispatch reported. They found the differences in intelligence between first- and later-born children disappeared when they considered the ages of their mothers when they had their first children.

It’s a family effect, not a birth-order effect, Wichman said.

Generally, the younger the mother when she has her first child the worse her children do on intelligence tests. Since women who start having their families young tend to have more children, that caused the apparent importance of birth order.

Not everyone agrees. Stanford University psychologist Robert Zajonc and other critics say that the sample is too small.