Intelligence and executive functioning in adult age: Effects of sibship size and birth order

Several studies have demonstrated that social influences from having been brought up in a family with few siblings and early in birth order result in higher scores on intelligence tests in childhood and adolescence as compared to having been brought up in large sibships. The present study examined whether influences of such social factors would have long-lasting effects on intelligence (block design and word comprehension) and executive functions (working memory and verbal fluency) in adulthood and old age, i.e., long after the individuals had moved out of the family structure they were born in. After having controlled for socioeconomic status and a variety of health conditions affecting cognition in adult life, a sibship size effect was demonstrated for executive functions but not for intelligence. The social influences of birth order affected only executive functions and working memory in particular; earlier born individuals performed better than later born individuals in tests assessing executive function but not in tests assessing intelligence. Implications for the relationship between executive functioning and intelligence, and implications for the Confluence and Resource Dilution Models (Blake, 1981; Downey, 1995, 2001; Zajonc, 1976) are discussed.