Effects of sibship size on intelligence, school performance and adult income: Some evidence from Swedish data

- Children with more older siblings have, on average, lower intelligence scores than children with fewer older siblings.
- Individuals with more younger siblings have, on average, lower incomes than individuals with fewer younger siblings.
- The effects cannot be explained by differences in backgrounds, such as SES, parental age, gender, and educational attainment.
- Sibship size has no effect on children's school grades when their intelligence is accounted for.

We examine the effects of child sibship size on intelligence, school performance and adult income for a sample of Swedish school children (n = 1326). These children were measured in grade three in 1965 (age 10) and in grades six (age 13) and nine (age 16), and the women and men were later followed up in adulthood at ages 43 and 47, respectively. Using Bayesian varying-intercept modeling we account for differences between school classes in each of our three response variables: IQ-scores, school grades and adult income, and control for background variables such as gender, socioeconomic status, and maternal- and paternal age. Consistent with previous research, we find patterns of decreasing IQ scores for increasing sibship sizes, specifically for an increasing number of older siblings. No relationships between sibship size and children's school grades are found. We find, however, patterns of decreasing adult income for an increasing number of younger siblings. In addition, considerable amounts of variations in intelligence scores as well as school grades are found between school classes. Some implications of the findings and suggestions for future research are provided.