Birth order is one of my favorite explanations for behavior. It's the rationale for why I'm bossy (I'm an oldest) and for why my older son Eli tends to be assertive and rule-conscious while my younger son Simon veers toward mischievous. Older children are supposed to be more aggressive and domineering, younger children more rebellious.
To my disappointment, however, studies suggest that the effects of birth order apply far more narrowly than I thought. Within a family, birth order matters (how could it not?). Older siblings throw their weight around and younger siblings either cower or figure out a way to defy them. But outside the home, the research shows that these patterns fall away and birth order has little effect on personality and self-expression. In one study, researchers watched siblings play and observed how the older ones run the show, "often by playing aggressively," as Judith Rich Harris recounts in her new book No Two Alike. Then they watched the same children playing with peers. The older siblings didn't dominate their peers more, and the younger siblings didn't dominate less. Similarly, studies of the effect of birth order on test scores, education, and earnings have collectively failed to account for differences in children's achievements.
Despite the claims of birth-order believers, the relationship between birth order and intelligence is, in the words of one respected researcher, a "methodological illusion."
There is only one sphere outside the home, it turns out, where there's solid proof that older siblings make their influence felt—the sphere of getting into trouble. According to a new study and the previous work on which it builds, older siblings make their mark by introducing their younger sidekicks to smoking, drugs, sex, and guns. Forget about older siblings as positive role models. What's far more prevalent, apparently, is premature exposure—pointing out the best spot to sneak a cigarette or buy beer underage.
The new study, conducted by a team led by Laura Argys, an economist at the University of Colorado at Denver, and published in the April issue of Economic Inquiry, found that younger siblings between the ages of 12 and 17 were more likely to smoke, drink, and smoke pot. Younger siblings between 14 and 17 were also more likely to be sexually active (and not use birth control). Younger brothers were more likely to have stolen and carried a gun. The researchers controlled for race, ethnicity, socioeconomics, and family size, leading the team to conclude that birth order, and not one of these other factors, made the difference. "Later-born children are especially prone to engaging in risky and delinquent behaviors," they wrote. Argys and her co-authors ventured that older siblings may initiate younger siblings to the various vices by bringing them along when they're hanging out with their older friends. Parents may also contribute to younger sibling delinquency, wittingly or unwittingly, by cracking down on firstborns but running out of the energy to do so when the later-borns hit their teens.
In my family, Eli is too young to buy Simon his first beer (ack, what a thought). But a mini-version of vice-mongering may already be unfolding. At 6, Eli's taste in DVDs has ushered in a shared brotherly love for Star Wars, which mostly means a love for light sabers, guns, and shoot-em-up games. Never mind that Simon, who is 3, watched all of 20 minutes of the movie. The point is that he has been introduced to weapons that Eli himself had only the fuzziest idea of when he was in preschool. Check the box for premature exposure, and also for parents falling down on the job. The kids don't have play weapons (OK, we did buy one plastic sword for Halloween). But Simon listens to Eli and his friends talk about guns and fashioned one out of the stand that holds up the Connect Four game. Eli isn't trying to corrupt his younger brother, if that's what all this play fighting is. But he does, just by being his older self.
In her 1998 book The Nurture Assumption, Judith Harris explosively argued that families matter a lot less than peers do for charting the course of a child's development. But she pointed out that the risky-behavior sibling research is the exception to the rule. "Teenagers like to hang around with their older siblings and their older siblings' friends," Harris writes, and that makes them part of the mix of peer influence that she sees as crucial. Birth order does matter when siblings' relationships spill over into the street, or the mall, or the car.
When I called Harris about Argys' study, she pointed me to the work of Joseph Lee Rodgers, a psychologist at the University of Oklahoma. In a 2003 paper, Rodgers also found that younger siblings are more likely to smoke at an early age than other children. His idea is that teen drinking and drug use and sex spread like contagious diseases and that older siblings are likely vectors of infection.
But the virus you catch from your older brother or sister doesn't necessarily make you act out forever. There's a second part to Argys' study, in which her team looked at data for younger siblings between the ages of 27 and 34. As adults, the younger siblings weren't more likely to drink and were only a bit more likely to smoke pot. Cigarette smoking seems to be the main effect of being a rebellious (or impressionable) younger sibling that lasts into adulthood, presumably because it's addictive. So go ahead and blame your big brother for your nicotine habit or your bong hits, but for all your other grown-up quirks and behavior, look somewhere else. For most of us, in the end, the reach of sibling influence is limited. Phew.