Abstract
The present study investigated the Danish secular trend of intelligence test scores among young men born between 1940 and 2000, as well as the possible associations of birth cohort changes in family size, nutrition, education, and intelligence test score variability with the increasing secular trend. The study population included all men born from 1940 to 2000 who appeared before a draft board before 2020 (N = 1,556,770). At the mandatory draft board examination, the approximately 19-year-old men underwent a medical examination and an intelligence test. In the statistical analyses, the IQ mean and standard deviation (SD) were estimated separately for each of the included annual birth cohorts based on information from birth cohorts with available total intelligence test scores for all tested individuals (i.e. 1940–1958 and 1987–2000; the mean and SD were interpolated for the intermediate birth cohorts). Moreover, the possible associations with birth cohort changes in family size, height as a proxy for nutritional status, education, and IQ variability were investigated among those birth cohorts for whom a secular increase in intelligence test scores was found. The results showed that the estimated mean IQ score increased from a baseline set to 100 (SD: 15) among individuals born in 1940 to 108.9 (SD: 12.2) among individuals born in 1980, since when it has decreased. Focusing on the birth cohorts of 1940–1980, for whom a secular increase in intelligence test scores was found, birth cohort changes in family size, height, and education explained large proportions of the birth cohort variance in mean intelligence test scores, suggesting that these factors may be important contributors to the observed Flynn effect in Denmark.
Highlights
MethodsThe possible associations with changes in family size, height, education, and IQ variability were investigated among those birth cohorts for whom a secular increase in intelligence test score was found (that is, the birth cohorts of 1940–1980)
Populations’ mean intelligence test scores have been rising since the beginning of the twentieth century
This study found that neither genetic selection favouring those with lower intelligence, accumulation of deleterious mutations due to rising ages at parenthood, nor replacement migration as a consequence of migrants with higher fertility and lower intelligence moving to countries with lower fertility and higher intelligence seem to have contributed to the observed secular decline, but that changes in test format and increasing sample selection are the most important contributors
Summary
The possible associations with changes in family size, height, education, and IQ variability were investigated among those birth cohorts for whom a secular increase in intelligence test score was found (that is, the birth cohorts of 1940–1980). The correlations of the birth cohorts’ average family size, height, and education with annual mean intelligence test scores were calculated based on the empirical observations and the same associations were estimated by use of three separate linear regression models as well as one combined model including all factors. The possible associations with changes in family size, height, education, and IQ variability were only investigated among those birth cohorts for whom a secular increase in intelligence test score was found as we in a previous study have looked at the possible explanations of the secular decline
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