Birth Order and Intelligence

This study examined the relationship between an individuals’ birth order and intelligence quotient (IQ) at 4th grade. Data were collected for 1,364 children in a study by the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development. Intelligence was computed using the Wechsler Abbreviated Scale of Intelligence (WASI), which assesses verbal, non-verbal, and general intelligence. The regression analysis was conducted using control variables of gender, mother’s education, a husband living in the home, and income-to-need ratio. Results indicated that mother’s education, a husband living in the home and birth order significantly predicted a child’s IQ. Birth order was negatively related to IQ scores in 4th grade, suggesting that latter-born children had lower IQ scores than children with earlier birth orders