Much hubbub has ensued from a Norwegian study published in the prestigious journal Science that purports to show that first borns enjoy a 2.3 point advantage in IQ. The study was noteworthy for its large sample size (more than 200,000 military conscripts) and for its innovative technique of examining individuals who had an elder sibling die in infancy to rule out "biological" explanations. Their conclusion is that intra-family social dynamics result in an intellectual advantage those men raised as eldest siblings.
Before American parents panic and sign up their second born children for extra tutoring, several notes of caution should be sounded. First, Norway is not the United States. Like other European countries, Norway has inherited a medieval culture of primogeniture, that is, of handing off the family farm (or other assets) to the eldest male child. This historical tradition, though not still practiced, may have generated tendencies within families that are quite distinct from how Americans--with a tradition of egalitarianism--raise their children.
Second, my own research finds that birth order is really a red herring in two-child families (for a summary, see here). It is only when the second born becomes the middle child in a family of three or more that birth order starts to matter. Folks of my generation will remember Jan Brady's plaintive cries of "It's always Marcia! Marcia! Marcia!" on 1970s show, The Brady Bunch. It turns out she had a legitimate point.
In fact, it has long been shown that in large families the last born tends to excel, particularly when there is a big gap between that child and the next oldest. Thus, even when studying siblings from the same family, birth order effects may depend on--or be confused with--family size effects. In fact, such effects would help reconcile the fact that previous studies have found that at younger ages, it is the latter borns that demonstrate an advantage: It could be the transition from being a second of two to a middle child that explains the disadvantage shown in the current study at an older age.
Third, Americans should take this 2.3 point different in perspective as compared to the many other factors within families that tend to matter more--such as birth spacing, gender, the timing of parental death or divorce, sexual orientation, skin tone and more. For example, I find that birth order effects can completely flip when a parental death or divorce occurs. In cases when a parent suddenly finds herself raising her kids on her own, she often leans heavily on the eldest child--particularly when she's a girl. She often has to shoulder burdens ranging from packing the lunchboxes of the younger siblings to sweeping the floors to even being an emotional support for the remaining parent. I call this the Cinderella effect; however, in real life, most siblings don't have a fairy godmother or magic pumpkin to rescue them. More often, these siblings make serious (and unappreciated) sacrifices for their younger brothers and sisters, who end up benefited at their expense. So birth order often matters, but how it matters depends on lots of other factors that vary within and across families.
Finally, in the same vein, it's worth keeping in mind that IQ is just one of many factors that predict success. We don't know how second borns fare on all those other factors (such as adaptability, emotional stability, perseverance, etc.) that the study didn't happen to measure. So, Americans with two chidren can relax about birth order. There are plenty more important things to worry about for our kids these days.