But the results also had the whiff of a paradox. If anything, later-born children should have a greater shot at success. By the time the second baby comes around, families are better prepared. They have more experience and more resources. So why do younger siblings lag behind?

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A pair of new studies points to an uncomfortable explanation: Maybe parents devote less time and care to the children who come later.

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In a paper released recently by the National Bureau of Economic Research, an international team of researchers analyzed a trove of data from two very different parts of the world: Denmark and Florida. In both places, they found the same patterns. Second-born children seemed to have the advantage at first. Compared to older siblings, they were healthier at birth and in early childhood. They attended schools of similar, if not better, quality.

And yet, by their teenage years, the younger siblings were more likely to get in trouble and tended to score lower on standardized tests.

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The differences were particularly stark for boys. In Florida and Denmark, second-born sons were about 30 to 40 percent more likely to get into trouble with the law compared to firstborn sons. They lagged behind about a third of a grade level in reading test scores. This could not be explained by differences between families or by general trends in test scores or delinquency.

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So the researchers started looking for clues that parents were treating siblings differently. In Denmark, where they had data on parental employment, they found that mothers spent slightly less time on maternity leave, were more likely to be working by the time their second-born sons were age 2. In both Florida and Denmark, younger siblings were more likely to be put into preschool programs — which is not a bad thing at all, especially if the programs were high-quality. But this hints at differences in the amount of time and attention parents were spending with their children at home.

Some of these disparities can be explained by the simple fact that firstborn children benefit doubly from maternity leave. Not only do firstborns begin their lives with undivided attention, they receive additional face time when parents stay home for the birth of their siblings. “It appears that the addition of a second-born child results in mothers spending more of a firstborn’s early childhood out of the labor force,” the researchers write.

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A related study published in November finds that American parents treat second-born children differently in other ways. Mothers are less likely to curb their drinking or smoking habits during their second pregnancies. After birth, they are less likely to breast-feed, and there’s some evidence that mother and fathers also become less attentive. As the researchers put it, it seems that “parents are choosing to relax what they might deem as nonessential rearing practices for their latter-born children.” Another paper from 2013 shows that American parents are often less strict with second- or third-born children.

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None of this rules out the other major theory for these differences, which involves the influence that siblings have on each other. Older brothers and sisters can serve as role models to emulate — or role models to rebel against. But it’s hard to tell what’s going on using the available data. What is remarkable is how consistent the second-born disadvantage is across the industrialized world. Thanks to the increasing availability of good data, the phenomenon has now been confirmed in several other countries, from the U.K. to South Korea.

Less-developed places show an opposite pattern. A 2014 study out of Ecuador showed that earlier-born children often didn’t do as well there. The researchers blamed differences in parental investment; they found that later-born children are more likely to be breast-fed, and receive more parental attention in early childhood. This was particularly true among poorer families. A 2010 report came to a similar conclusion after analyzing data from a dozen countries in sub-Saharan Africa: In poorer households, later-born children receive better educations, while in richer households, the earlier-born children had the advantage.

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What all these studies have in common is that they point to the vital importance of parents, particularly at the start of life. Even within the same household, children can turn out quite differently, and at least some of their fate seems to be tied to how they were treated as toddlers.

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