Galton Lecture. Behaviour genetic studies of intelligence, yesterday and today: the long journey from plausibility to proof

The Wilson Effect: The Increase in Heritability of IQ With Age

Ronald Wilson presented the first clear and compelling evidence that the heritability of IQ increases with age. We propose to call the phenomenon ‘The Wilson Effect’ and we document the effect diagrammatically with key twin and adoption studies, including twins reared apart, that have been carried out at various ages and in a large number of different settings. The results show that the heritability of IQ reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood. These conclusions apply to the Westernized industrial democracies in which most of the studies have been carried out.

How do educational attainment and gender relate to fluid intelligence, crystallized intelligence, and academic skills at ages 22-90 years?

Educational attainment and gender differences on fluid intelligence (Gf), crystallized intelligence (Gc), and academic skills in reading, math, and writing were analyzed for stratified adult samples ranging in age from 22 to 90 years. The data sources were the adult portions of the standardization samples of the second editions of Kaufman Brief Intelligence Test (N = 570) and the Kaufman Test of Educational Achievement-Brief Form (N = 555). Five univariate analysis of covariance were conducted with age as the covariate. Correlational analysis supplemented the covariate analyses to better understand the relationship of the five variables to education. All variables related significantly and substantially to years of formal schooling, an important finding in view of the key nature of this background variable for conducting neuropsychological assessments, as elaborated by Heaton and his colleagues. Surprisingly, Gf related just as strongly to education as did the school-related Gc. Among academic skill areas, math correlated higher with years of formal schooling than did either reading or writing. Women significantly outperformed men on the writing test and the reverse was true for the math test; other gender differences were not significant. These analyses fill a gap in the literature regarding the nature of gender and education differences in academic skills for heterogeneous samples of normal adults between young adulthood and old age and have practical implications for neuropsychological assessment.

Reared apart Korean female twins: Genetic and cultural influences on life histories, physical and health-related measures, and behavioral traits

This paper reports physical and behavioral similarities and differences in a pair of 18-year-old monozygotic (MZA) female twins, raised apart from birth in dramatically different cultures: South Korea and the United States. The main outcome measures included life history characteristics (e.g., age at separation, age at reunion), physical and health-related traits (e.g., height, weight, body mass index, age at menarche, illnesses), and behavioral traits (e.g., IQ, special mental abilities, personality traits, dietary preferences). Similarities in some physical characteristics, as well as differences, were noted. High levels of concordance were obtained for IQ and for two special mental ability measures. Selected personality traits presented a mixed picture with respect to concordance and discordance, and dietary preferences showed some unexpected similarities. The findings are discussed with reference to genetic and cultural influences on development.

Intelligence Correlations Between Brothers Decrease With Increasing Age Difference

Research on twins and unrelated persons living together indicates that, although differences in intelligence between persons are caused by both genetic and environmental factors, similarities—and especially those between adults—are mostly or exclusively due to shared genes. However, twin and adoption designs have been criticized for incorrectly estimating the effects of the environment on the similarity between relatives, and studies of environmental effects in other family constellations may be enlightening. In a sample comprising more than 334,000 pairs of brothers (ages 18–21 years) who were tested at the mandatory conscription for military service in Norway, correlations between brothers' intelligence-test scores decreased with increasing age difference between the brothers. This result indicates that family environmental factors have an impact on the intelligence of young adults. Analyses of subgroups defined by family size and parental education demonstrated that the effects of age difference are essentially independent of these factors.