A within-family analysis of birth order and intelligence using population conscription data on Swedish men

This study examines the relationship between birth order and intelligence in Sweden. This research question has been of interest for decades, but only one study using a sibling comparison design has found that birth order has a negative effect on intelligence. The data used in this study is Swedish administrative register data, with data on cognitive ability drawn from the military conscription register for men born 1965 to 1977. Within-family comparison linear regression models are used to estimate the difference in cognitive ability by birth order amongst brothers. I find that there is a negative relationship between birth order and cognitive ability. This is consistent in sibling-group-size-specific analyses of sibling groups with two through to six children. Further analyses demonstrate that this negative relationship between birth order and intelligence is consistent in different socioeconomic status groups, and amongst individuals born in the 1960s and 1970s. Analyses of brothers in two-child sibling groups show that the relationship between birth order and intelligence varies by the birth interval. Second borns have a statistically significantly lower cognitive ability score if the birth interval is up to six years, but not if it is longer