Intelligence in fathers and sons: Familial resemblances and generational differences

Intelligence test scores, educational levels, and height were obtained from Danish draft board records for three groups of fathers and sons: 25 biological fathers with sons who were adopted away, 17 stepfathers with stepsons, and 29 fathers with sons in intact families. Father-son correlations for intelligence suggest a modest heritability with little familial environmental effect, although a comparison of mean levels reveals some familial environmental effect for both intelligence and educational level. However, the correlations for educational level and height are somewhat inconsistent. In combination with earlier reported data on siblings, the present results may indicate genetic dominance effects for intelligence. Normative data show that the mean intelligence test score has risen between the paternal and the filial generations, possibly as a result of raised educational levels.This study was supported in part by National Institute of Child Health and Human Development Grant 1-HD-82807.