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Hegelund ER, Teasdale TW, Okholm GT, et al. The secular trend of intelligence test scores: The Danish experience for young men born between 1940 and 2000. PLoS One. 2021;16(12):e0261117doi: 10.1371/journal.pone.0261117.
Hegelund, E. R., Teasdale, T. W., Okholm, G. T., Osler, M., Sørensen, T. I. A., Christensen, K., & Mortensen, E. L. (2021). The secular trend of intelligence test scores: The Danish experience for young men born between 1940 and 2000. PloS one, 16(12), e0261117. https://doi.org/10.1371/journal.pone.0261117
Hegelund, Emilie R, et al. "The secular trend of intelligence test scores: The Danish experience for young men born between 1940 and 2000." PloS one vol. 16,12 (2021): e0261117. doi: https://doi.org/10.1371/journal.pone.0261117
Hegelund ER, Teasdale TW, Okholm GT, Osler M, Sørensen TIA, Christensen K, Mortensen EL. The secular trend of intelligence test scores: The Danish experience for young men born between 1940 and 2000. PLoS One. 2021 Dec 09;16(12):e0261117. doi: 10.1371/journal.pone.0261117. eCollection 2021. PMID: 34882746; PMCID: PMC8659667.
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PLoS One. 2021 Dec 09;16(12):e0261117. doi: 10.1371/journal.pone.0261117. eCollection 2021.

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

PloS one

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

Affiliations

  1. Department of Public Health, University of Copenhagen, Copenhagen, Denmark.
  2. Department of Psychology, University of Copenhagen, Copenhagen, Denmark.
  3. Center for Clinical Research and Prevention, Bispebjerg and Frederiksberg Hospitals, Frederiksberg, Denmark.
  4. Department of Public Health, University of Southern Denmark, Odense, Denmark.

PMID: 34882746 PMCID: PMC8659667 DOI: 10.1371/journal.pone.0261117

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.

Conflict of interest statement

The authors have declared that no competing interests exist.

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