Second-born boys are roughly 25 to 40 percent more likely to face school discipline or to enter the criminal justice system than their older brothers, according to an analysis of family records from Denmark and the state of Florida by economists Joseph Doyle, Sanni Breining and colleagues, circulated in 2017 and updated as a National Bureau of Economic Research working paper. The pattern held across two very different societies, across income brackets, and after the researchers controlled for the usual suspects like family income, mother’s age at birth, and education.
That is a striking number. It is also, the authors say, an average across hundreds of thousands of families, not a verdict on any child. The interesting question is what could explain a gap that big, and what parents can do with it.
What the study actually found
The Breining team pulled administrative data on more than 285,000 brothers from Denmark and tens of thousands more from Florida. They focused on brothers because comparing siblings within the same family controls for the confounders that bedevil parenting research. Same parents, same neighborhood, often the same school. The only thing that differs systematically is the order they arrived in.
What they saw, repeatedly, was that second-born boys had higher rates of school suspension, of juvenile delinquency contacts, and of adult incarceration than first-borns. The gap showed up at every income level. It was not driven by a small number of troubled families dragging the average around. Crucially, the same gap did not appear, or appeared much more weakly, between first- and second-born girls.
That last detail matters because it pushes back on the simplest explanation, which would be that parents just get tired and stop parenting as carefully the second time. If exhaustion alone were the story, you would expect daughters to drift the same way. They did not. Whatever is going on seems to interact with being a boy in particular.
Why might second-born boys drift?
The viral version of this story boils the explanation down to two ideas: parents pay less attention the second time around, and second-borns model themselves on an older sibling who is, as one Facebook post put it, “a toddler making, well, toddler decisions.” Neither idea is new. Both have research behind them, though the picture is messier than a caption can fit.
On the parental investment side, an analysis by economists V. Joseph Hotz and Juan Pantano found that parents really do behave differently with later-born children. Using survey data from American families, they showed that mothers monitor first-borns more strictly, push them harder on schoolwork, and dish out tougher consequences for slipping grades. With second and third children, the same parents loosened up.2 Hotz and Pantano framed this as a strategic move. By being seen to come down hard on the eldest, parents establish a reputation that does some of the disciplinary work for them with the kids who follow. It is a tidy theory, and the data line up with it.
The role-model angle has its own backing. Older siblings exert a measurable influence on younger ones. A first-born mostly imitates adults, who are more regulated than three-year-olds. A second-born watches a small human still figuring out impulse control in real time. Some of that gets absorbed.
The undivided-attention years
Anyone with a second child knows the time math changes overnight. The first baby gets the unhurried hours, the eye contact during feeds, the parent who talks to them about the cat and the weather. The second baby gets a parent also wiping a toddler’s nose and refereeing a Lego dispute.
Why does that early window matter? Because it lines up with brain development when language, executive function, and emotional regulation are wiring in at speed. Children who hear more words and have more back-and-forth conversation in the first three years tend to score higher on vocabulary, attention, and self-control years later. Across populations, attention in the early years shows up as small advantages that compound.
The Breining team did not measure parental talk directly, but they did show that parents reported being less involved with second-born sons in the school years. They were less likely to know who their child’s teacher was, less likely to attend school events, less likely to enforce homework rules.2 Those are not enormous differences in any one family. Across an entire population, they add up.
It is not just one study
If the Breining paper sat alone, it would be easy to wave away. It does not. Sandra Black, Paul Devereux, and Kjell Salvanes have spent years mining Norwegian birth and tax records and have repeatedly found that birth order predicts educational attainment, earnings, and health into adulthood, with first-borns coming out marginally ahead.3 Their later-borns are not doing badly in any absolute sense. They simply track a little behind their oldest siblings on average.
On the personality side, the picture is more complicated. A widely cited 2015 study by Julia Rohrer and colleagues, published in the Proceedings of the National Academy of Sciences, looked at more than 20,000 adults across Germany, the United States, and the United Kingdom and found a small bump in measured intelligence for first-borns but essentially no birth-order effect on the Big Five personality traits like extraversion, agreeableness, or conscientiousness.1 So if you grew up convinced that your older sister is bossy because she is the older sister, the data say maybe, and maybe you just have a bossy sister.
A separate study of monozygotic twins reared apart in different birth-order positions in their adoptive families found very little support for the idea that being raised as the “first” or “second” reshapes personality.4 If birth order had a strong direct effect on who a person becomes, you would expect twins placed in different sibling slots to diverge. They mostly did not.
So the picture from the literature is narrower than the viral version. Birth order does not turn first-borns into responsible accountants and second-borns into rebels. What it appears to do, in some populations and especially among brothers, is produce small but real gaps in outcomes measured by school systems and courts.
The brother problem
Why is the gap most visible between brothers? Several explanations are on the table, and none of them is settled.
One is that boys, on average, are more sensitive to differences in adult attention and structure during the early years. That fits a broader pattern in developmental research: when family environments wobble, whether through divorce, parental stress, or low income, boys tend to show the wobble in their behavior more readily than girls. They are not more fragile in any deep sense. They just appear to externalize more, meaning their distress shows up as acting out rather than as internalized worry.
Another is that older sisters and older brothers play different roles for a younger boy. A new meta-analysis published in Psychological Bulletin in 2025 found that across many studies, parents tend to give daughters slightly more warmth and structure than sons, and that gender shows up as one of the strongest predictors of how parents differentiate between their kids.5 If that pattern is real, a second-born boy with an older sister is in a different ecosystem than a second-born boy with an older brother. The Breining data hint at this. The brother-on-brother gap was the largest.
A third possibility is that some of what looks like a behavior gap is actually a gap in how schools and the justice system respond. Teachers and police officers are people, and people sometimes notice the second kid in a family by the older one’s reputation, for better or worse. The researchers tried to net this out, but it is hard to fully strip school-level effects from a study like this.
What this is not
This is not a finding that second-born boys are doomed. The 25 to 40 percent figure is a relative gap, and the absolute rates of school discipline or juvenile contact in any given family are still small. Most second-born boys, in Denmark and Florida and everywhere else, sail through school and adulthood without showing up in any of these statistics. The study identifies a tendency in the aggregate, not a fate.
It is also not a clean parenting verdict. The data are consistent with several stories. Maybe parents really do under-invest in second-borns. Maybe second-borns drift toward rebellion because the niche of “responsible eldest” is taken, an idea the historian Frank Sulloway proposed decades ago with a thin and contested research record since.4 Maybe schools and courts treat younger brothers differently. Maybe all three are partly true.
And it is not a license to start treating a one-year-old as a future statistic. Parenting research that finds an average effect across hundreds of thousands of children rarely tells you anything actionable about your specific kid. What it can do is gently flag where attention might pay off.
What can parents actually do with this?
If there is a takeaway, it is the one buried in the Hotz and Pantano work, not the one in the headline. The behaviors that seem to matter, knowing your child’s teacher, enforcing homework rules, monitoring how time is spent, are all things a parent can choose to keep up with the second child even when life has gotten louder. They cost effort, not money.
Carving out one-on-one time with a second-born can feel forced, and the windows are small. Fifteen minutes of reading at bedtime, a short walk, a car ride with the radio off and an actual conversation. These add up over years in ways that look invisible week to week.
It is also worth being honest about the older sibling. A first-born is not a co-parent. A four-year-old is not equipped to model emotional regulation for a two-year-old, and pretending otherwise puts pressure on both. Spending time with the second child outside the gravitational pull of the first changes what they imitate.
Family income, neighborhood, school quality, temperament, and a hundred other factors swamp birth-order effects in any individual case. But within a family, where those factors are held constant, small differences in how the second child is parented are the lever the research keeps pointing at.
Common questions about birth order and behavior
Does birth order really predict personality?
Mostly no. The largest careful studies, including Rohrer and colleagues in 2015, find very small or zero effects on Big Five personality traits like extraversion or agreeableness.1 A small first-born advantage on measured intelligence shows up more reliably, but it is on the order of one or two IQ points.
So is the second-born-boys-and-trouble finding real?
The data are real. The Breining team found a 25 to 40 percent higher rate of school discipline and justice-system contact for second-born boys versus first-born boys in Denmark and Florida. The interpretation is still debated. It could be parental attention, sibling modeling, school treatment, or some mix.
Does this apply to girls too?
Less clearly. The gap was much smaller, sometimes statistically indistinguishable from zero, between first- and second-born girls. The pattern shows up most strongly between brothers.
Does the age gap between siblings matter?
It might. Other research on Norwegian families suggests that very tight age spacing, under about two years, is associated with slightly worse educational outcomes for the younger child, possibly because parental attention is stretched thinner.3 The effects are small.
What is the single most useful thing a parent can take from this?
Probably this. Whatever monitoring and one-on-one time you naturally gave the first child, intentionally protect some of it for each child after that. The cost is mostly attention. The payoff shows up years later in places that are hard to see at the time.
The honest version
The viral framing of this research, that second-borns are chaos agents, makes a satisfying caption and a misleading headline. The careful version is smaller and, for parents, more useful. There is a real, replicated gap between first-born and second-born boys in some outcomes schools and courts measure. The gap is consistent with parents subtly changing how they parent the second child, and with younger brothers learning early behavior from an older brother whose own behavior is still under construction.
None of that condemns a child. Most second-borns do fine. Plenty of first-borns do not. What the research offers is a quiet nudge, that the boring work of attention, structure, and one-on-one time is worth fighting to preserve as a family grows, even when the laundry pile and the toddler’s tantrum suggest otherwise.
Sources
- Rohrer JM, Egloff B, Schmukle SC. Examining the effects of birth order on personality. Proceedings of the National Academy of Sciences. 2015. PubMed: 26483461
- Hotz VJ, Pantano J. Strategic parenting, birth order, and school performance. Journal of Population Economics. 2015. PubMed: 26366045
- Black SE, Devereux PJ, Salvanes KG. Healthy(?), wealthy, and wise: Birth order and adult health. Economics and Human Biology. 2016. PubMed: 27442721
- Segal NL. Personality and birth order in monozygotic twins adopted apart: a test of Sulloway’s theory. Twin Research and Human Genetics. 2008. PubMed: 18251684
- Jensen AC, Jorgensen-Wells MA. Parents favor daughters: A meta-analysis of gender and other predictors of parental differential treatment. Psychological Bulletin. 2025. PubMed: 39818912
