Does Birth Order Have an Effect on Intelligence

In 1874 Francis Galton reported that firstborn children were overrepresented as high achievers in various scientific fields. There were flaws in Galton’s methodology, for instance, he did not count female children in his results. Male subjects were counted as a first born even if they were the tenth child, but the nine older siblings were female (Esping, 2003). However, Galton’s conclusion that birth order correlates with intelligence and academic attainment remains popular. Even in the last decade, other researchers, in both Europe and North America, have confirmed and reasserted Galton’s conclusion.
What studies have demonstrated that birth order influences intelligence and/or achievement?
Research by Christensen and Bjerkedal concluded that birth order has a small impact on educational attainment (Christensen & Bjerkedal, 2010). That conclusion has also been reported by other related studies. Analysis of the National Longitudinal Survey of Youth (NLSY) and the Wisconsin Longitudinal Study (WLS) show that birth order has an effect on educational attainment and intelligence (Retherford & Sewell, 1991 and Rodgers, Cleveland, van den Oord & Rowe, 2000). Also, earlier research on Norwegian male military conscripts also demonstrated that birth order impacts on intelligence (Bjerkedal et al., 2007). The confluence model theorizes that first born children are raised in an adult oriented, highly intellectual environment. Also, when first born children interact with their younger they adopt the role of teacher. This is known as the tutor effect (Zajonc& Sullaway ,2007).

Are studies that support birth order effect on intelligence and/educational attainment flawed?
Wichman, Rodgers and MacCallum suggest a critical flaw in previous research that supports that birth order has an effect on intelligence and/or educational attainment They suggest that in larger families the first born is equally intelligent as the fourth-born child, but they are not as intelligent as children from a smaller family (Wichman et al,2006). The studies that demonstrate a link between educational attainment and/or intelligence and birth order have been criticized by other researchers. However, according to the confluence model it is only as children with younger siblings approach adulthood that they finally achieve maximum benefit from teaching their younger siblings, as it typically increases their efforts to do well scholastically (Zanjonc & Sulloway, 2007).
What factors other than birth order influence intelligence and or achievement? Wichman, Rodgers and MacCallum argue that the findings were a result of differences between families, not within families. They suggest that the younger a mother is at the birth of her first child will result in lower intelligence scores within the family. Younger mothers tend to be less educated, have more children and lower income. When researchers controlled for mother’s age at first birth, the effect on birth order on intelligence was nearly eliminated. In their opinion birth order appears to have an effect on intelligence, but that’s only because larger families don’t have the advantages of smaller families. Family environment and genetic influences are the most important factors and they may override birth order (Wichman et al., 2006).