First, middle, and only children will probably never stop debating the merits of their birth order. But a few things are pretty well settled. Firstborns in developed countries tend to earn more, have higher IQs, and end up more educated. And there’s evidence parents are biased toward them. But there’s no clear advantage when it comes to health over the long run. It’s complicated, but in a lot of ways, it’s worse to be born first, according to a new NBER working paper (paywall).
First-borns tend to be significantly less healthy when it comes to major physical markers. They’re more likely to be overweight or obese, to have high blood pressure, and to have high triglycerides, all of which are highly associated with heart disease.
But being born later has its consequences, too. Later-borns are more likely to smoke, an effect that increases further down the birth order. Firstborns are 5% less likely to smoke than second-borns, and 13% less likely than fifth-borns. Later-borns also self-report worse overall physical and mental health, and happiness.
These effects persist regardless of differences in earnings and education.
This is one of the first truly robust studies to examine health and birth order. Birth order research is usually somewhat unreliable because later-born children (by definition) come from larger families, which skews the data. This study used a large and long running Norwegian dataset, and is able to compare children from within the same family, controlling for things like family size, parental age, and age differences between siblings.
There are a lot of possible reasons for these differences, from the biological (the maternal immune system may change with the number of children, for example), to personality-based and environmental.
It’s impossible to account for all these factors. But the researchers were able to look into a couple areas for which they did have data: smoking and breastfeeding.
Mothers are about 4% more likely to smoke at the beginning of their first pregnancy than later ones. But they’re more likely to quit during that first pregnancy than they are subsequent ones. Overall, later-borns are less likely to be exposed to smoke. But parents seem to change behavior more to benefit firstborns.
Firstborns are also breast-fed for longer, again showing parents invest in children differently depending on birth order.
Though the data is mixed, it does give some hope to later-born children who face a pretty intimidating set of data suggesting they’re less likely to thrive.