Deriving personality from birth may instinctively strike you—as it has always struck me—as pseudo-Freudian cleverness barely passing for empiricism. But Adler’s hypotheses have held up in numerous studies. In a 2013 paper, "Strategic Parenting, Birth Order and School Performance," V. Joseph Hotz, a professor of economics at Duke University, and Juan Pantano, a professor of economics at Washington University in St. Louis, used American data to show that school performance declines with birth order.

The researchers chalked their findings up to what they called the “reputational model of strategic parenting.” Put simply, parents invest a lot of time establishing rules for their first child, building a reputation for toughness that they hope will trickle down to later children. As a result, first-borns are doubly blessed—lavished with their parents’ attention, and then entrusted to act as the rules enforcer of the family, which builds intelligence, discipline, and leadership qualities. In surveys, parents report that they consider their older children more successful, and they are less likely to discipline their later-born children for infractions, such as acting up or not doing homework.

This new study relies on Swedish data, and it comes to a similar conclusion. First-borns aren’t just healthier or smarter, but also they score higher on “emotional stability, persistence, social outgoingness, willingness to assume responsibility and ability to take initiative.” The researchers ruled out genetic factors; in fact, they uncovered evidence that later-born children might be healthier than first-borns.

Instead, the differences among siblings had everything to do with family dynamics in the children’s early years. First, having more children means parents can spend less time on each child, and as parental investments decline, so may IQ.

Second, the most important effect, the researchers said, might not be “strategic parenting” but something more like “strategic brothering.” As siblings compete for their parents’ love (or ice cream, or toys), they occupy certain niches—older siblings demonstrate their competence and power, while younger siblings cultivate more creative strategies to get attention. This effect seems particularly strong among later-born boys with older brothers. Younger brothers are much more likely to enter “creative” occupations— like architect, writer, actor, singer, or photographer—if they have older brothers, rather than older sisters. In other words, among young brothers, specialization within the family forecasts specialization in the workforce.

There is a sneakily profound implication in this idea that family dynamics during childhood can shape adult personality. Young children are exquisitely sensitive to their environment, in ways that often have lasting effects. On the negative side, early exposure to pollution or lead can depress education attainment and lead to juvenile delinquency and violence. On the positive side, cash assistance for children doesn't just cut child poverty—which, alone, would be an important achievement—but also seems to increase college enrollment and earnings in adulthood. In a 2015 paper “The Long-Term Effects of Early Life Medicaid Coverage,” economists concluded that mothers with access to prenatal coverage under Medicaid later had children with lower obesity rates, higher high-school graduation rates, and higher incomes in adulthood. At the federal level, where Washington spends $7 on seniors for every $1 on young children, there is a gerontocratic bias toward crafting and tweaking policy for old people. But perhaps the first-borns in Washington should pay more attention to babies.