A recent NYT article discusses studies finding that the first-born in a family has a higher IQ than that child’s siblings. It makes some sense; as the article points out:

Firstborns have their parents’ undivided attention as infants, and even if that attention is later divided evenly with a sibling or more, it means that over time they will have more cumulative adult attention, in theory enriching their vocabulary and reasoning abilities.

What researchers can’t figure out is why, among kids under 12, the younger siblings outscore their older sisters and brothers on IQ tests. One theory:

Adding a young child may, in a sense, diminish the family’s overall intellectual environment, as far as an older sibling is concerned; yet the younger sibling benefits from the maturity of both the parents and the older brother or sister. This dynamic may quickly cancel and reverse the head start the older child received from his parents.

See, this is why we can’t risk having a third kid, despite how much fun some people make it sound. We just can’t risk diminishing our overall intellectual environment any further…