A Multilevel Approach to the Relationship Between Birth Order and Intelligence

Abstract

Many studies show relationships between birth order and intelligence but use cross-sectional designs or manifest other threats to internal validity. Multilevel analyses with a control variable show that when these threats are removed, two major results emerge: (a) birth order has no significant influence on children's intelligence and (b) earlier reported birth order effects on intelligence are attributable to factors that vary between, not within, families. Analyses on 7- to 8 - and 13- to 14-year-old children from the National Longitudinal Survey of Youth support these conclusions. When hierarchical data structures, age variance of children, and within-family versus between-family variance sources are taken into account, previous research is seen in a new light.

Education and Cognitive Ability as Direct, Mediating, or Spurious Influences on Female Age at First Birth: Behavior Genetic Models Fit to Danish Twin Data

“…But differences between families, especially in education, do result in differences in AFB. This finding parallels research locating the family structure-intelligence relationship in betweenfamily variance, rather than the (long-believed) within-family interpretation of this relationship (Wichman et al 2006). …”

“…The behavior genetic analysis is more nuanced and helps us locate the source of the causal relationships, because the design accounts for, and the data potentially contain, both within-and between-family variance. Only when between-and withinfamily variance are separated and explicitly modeled can we logically identify and attribute causal explanation separately to these two sources (see also Wichman et al 2006).…”

“…Second, previous empirical results suggest that within low-fertility countries, the onset of childbearing may be a more relevant biosocial measure of fertility than number of children (e.g., Kohler, Rodgers, and Christensen 1999), because of its substantial variation across individuals, its strong connection to education and human capital accumulation (especially for women), and its strong empirical link to the second demographic transition framework. Finally, AFB is often associated with other features of the family environment, many of which are important aspects of demographic transition theory, including socioeconomic status (SES), parental education, and parental IQ (see, e.g., Wichman, Rodgers, and MacCallum 2006).…”

“…In other words, birth order (a variable measuring differences between children within families) has been a proxy within cross-sectional data sets for many between-family differences that appear to be the actual causal agents (e.g., SES, education, parental IQ, etc.). When IQ patterns were studied among siblings within families in the NLSY (and other data sets as well), the negative birth order-intelligence relationship substantially weakened or even disappeared (e.g., Rodgers 2001;Wichman et al 2006). Within the current study, we will also distinguish between these two sources of variability to identify whether the emerging patterns have within-family status or rather reflect differences between families (or possibly both).…”

Smoking during pregnancy and offspring externalizing problems: An exploration of genetic and environmental confounds

“…There is a history of using the NLSY79 to explore causal mechanisms using differences within and between families, the theoretical basis of the approach used in this article. For example, researchers have used the dataset to explore the relation between birth order and intelligence (Wichman, Rodgers, & MacCallum, 2005), maternal alcohol and illicit drug use and offspring psychopathology (Chatterji & Markowitz, 2001), teenage childbearing and the women's later adjustment (Geronimus & Korenman, 1992), and maternal age at first birth and offspring adjustment (Turley, 2003).…”