The results from the best of these papers generally show that the number of children plays a relatively little role in determining schooling or IQ.2 They do find that birth order matters. Later-born children tend to do (slightly) worse on IQ tests and get less schooling than their earlier-born siblings. This may be due to parents having less time and resources to devote to them. But it’s not the number of children that drives the association. A firstborn child with two siblings seems to do the same as a firstborn child with one.
