Journal Article

The secular trend of intelligence test scores: The Danish experience for young men born between 1940 and 2000

Mortensen, Erik L., Teasdale, Thomas W., Okholm, Gunhild T., Sørensen, Thorkild I. A., Osler, Merete, Hegelund, Emilie R., Christensen, Kaare

2021

Overview

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.

• Publisher: Public Library of Science,Public Library of Science (PLoS)
• Subject: Adult / Biology and Life Sciences / Birth Cohort / Body Height / Cognition - physiology / Cohort analysis / Cohort Studies / Demographic aspects / Denmark / Education / Educational Status / Family size / Female / Fertility / Flynn effect / Forecasts and trends / Government archives / Health aspects / Humans / Intellect / Intelligence / Intelligence - physiology / Intelligence levels / Intelligence tests / Intelligence Tests - statistics & numerical data / Male / Mean / Measurement / Medicine and Health Sciences / Military service / Nutrition / Nutritional status / People and Places / Physical examinations / Population / Population studies / Psychological aspects / Public health / Research and Analysis Methods / Social aspects / Social Sciences / Statistical analysis / Systematic review / Time Factors / Variability / Young Adult / Young men