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Cited by 12Cognitive Abilities and TestingIntergenerational and Educational Inequality StudiesHistorical Economic and Social Studies

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

Open access
Hegelund, Emilie R., Teasdale, Thomas W., Okholm, Gunhild T., Osler, Merete, Sørensen, Thorkild I. A., Christensen, Kaare, Mortensen, Erik L.·2021·PLOS ONE
DOI 10.1371/journal.pone.0261117PMC PMC8659667
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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.

AI snapshot

Written from the paper’s full text.

TL;DR

This Danish register-based cohort study estimated the secular trend in intelligence test scores among men born 1940–2000 and found mean IQ rose from 100 (SD 15) for the 1940 birth cohort to 108.9 (SD 12.2) for the 1980 cohort, followed by decline. Among birth cohorts 1940–1980, changes in family size, height as a nutrition proxy, and education explained large proportions of the variance in mean intelligence scores.

Key findings

  • Estimated mean IQ increased from 100 (SD 15) among men born in 1940 to 108.9 (SD 12.2) among men born in 1980, then decreased; average gains were 3.8 IQ points for 1940–49, 2.5 for 1950–59, 1.3 for 1960–69, and 0.3 for 1970–79, followed by average decreases of 0.6 for 1980–89 and 1.3 for 1990–99.
  • The SD declined from 15.0 for the 1940 cohort to 10.0 for the 2000 cohort, and the correlation between birth cohorts’ mean and variance in intelligence scores was r = -0.98 (p<0.001).
  • For birth cohorts 1940–1980, cohort-level correlations with mean intelligence scores were r = -0.75 for family size, r = 0.97 for height, and r = 0.99 for education; collectively these three factors explained 99.6% of the variance in mean intelligence scores.
  • Individual-level associations were weaker: r = -0.06 for family size, r = 0.26 for height, and r = 0.66 for education, with R² values of 0.014, 0.067, and 0.487, respectively.

Methods

  • Register-based cohort study of all Danish men born 1940–2000 who appeared before a draft board before 2020 (N = 1,556,770); at the mandatory draft board examination, approximately 19-year-old men underwent a medical examination and the Børge Priens Prøve intelligence test, a 0–78 total score comprising logical, verbal, numerical, and spatial subtests.
  • Only birth cohorts 1940–1958 and 1987–2000 had total intelligence test scores available for all tested conscripts; IQ means and SDs were estimated separately for each birth cohort using linear regression models with linear, quadratic, and cubic birth year terms, and values for intermediate cohorts were interpolated. The 1940 cohort was standardized to mean 100 and SD 15.
  • For birth cohorts 1940–1980, where a secular increase was found, the study examined cohort- and individual-level associations of family size, height, education, and IQ variability with intelligence test scores using correlations and linear regression models, including a combined model and a model additionally including birth cohort.

Limitations

  • The paper states that only total intelligence test scores were available, so it could not evaluate whether the intelligence construct changed over time or whether trends differed across subtests or by g-loading; only some birth cohorts had total scores for all tested conscripts, requiring interpolation for the remaining cohorts.
  • The paper states that the investigation of contributors may have been influenced by which birth cohorts had available scores, with sensitivity analysis showing family size differed when using only 1940–1958 versus all cohorts through 1980; rising exemption from draft board examinations may have artificially inflated means in younger cohorts.
  • The paper states that the correlational design cannot provide causal evidence and that explained variance should not be interpreted as causal influence; it also notes the secular trend may be limited to young people born in the same time and place as the study population, though the explanatory factors may be more generalizable.

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Integrity signals

Registry

Crossref · 10.1371/journal.pone.0261117

Cites

36 of 36 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

7 of 66 cited sources checked · 2 supported · 0 flagged · accuracy not yet measured

  • Premisecitation

    Nutrition, particularly high-quality protein, influences body height, head circumference, and intelligence.

    “Improved prenatal and postnatal nutrition may also have contributed to the secular increase in intelligence test scores as nutrition, particularly high-quality protein, influences the makeup of the human body, such as body height, head circumference, and intelligence [20, 23–25] and we observed simultaneous increases in body height and intelligence.”
    Quote in the paperSubstantial claim1 of 4 sources checked
    • cites [20] JM Perkins, 2016, Adult height, nutrition, and population health, Nutr Rev source

      The available Abstract-labeled passages contain keywords and bibliographic information, not findings about nutrition’s effects. Searches for high-quality protein, head circumference, and intelligence found no passage in our copy that addresses the claim. (LLM, conf=0.72)

      Experimental — accuracy not yet measured
    • cites [23] R. Lynn, 1989, A nutrition theory of the secular increases in intelligence; positive correlations between height, head size and IQ, British Journal of Educational Psychology source
    • cites [24] R. Lynn, 1990, The role of nutrition in secular increases in intelligence, Personality and Individual Differences source
    • cites [25] R. Lynn, 2009, What has caused the Flynn effect? Secular increases in the development quotients of infants, Intelligence source
  • Findingown data

    The mean height increased from 175 cm among those born in 1940 to 180 cm among those born in 1980.

    “Meanwhile, the mean height increased from 175 cm among those born in 1940 to 180 cm among those born in 1980”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.
  • Findingown data

    The empirical data show a true decline of 0.4 IQ points from those born in 1990 to 1991.

    “However, looking at the empirical data, it is clear that the estimated change for the last birth decade conceals a true decline of 0.4 IQ points from those born in 1990 to 1991”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.
  • Findingreasoning

    The observed association between family size and intelligence was considerably larger in cohort-level analyses than in individual-level analyses.

    “This may explain why the observed association between family size and intelligence was considerably larger in our cohort-level analyses than in our individual-level analyses.”
    Quote in the paperNot a substantial claim
  • Findingown data

    There was still a significant association between birth cohort and intelligence test scores when birth cohort changes in family size, height, and education had been accounted for (p<0.001).

    “Interestingly, there was still a significant association between birth cohort and intelligence test scores when birth cohort changes in family size, height, and education had been accounted for (p<0.001).”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.
  • Classified as probably not a substantial claim. 27
  • May combine two claims. 42

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The Flynn effect amounted to about three IQ points per decade during the past century.

    “Two large meta-analyses have found that the Flynn effect amounted to about three IQ points per decade during the past century [4, 5].”
    Quote in the paperNot a substantial claim1 of 2 sources checked
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source
    • cites [5] LH Trahan, 2014, The Flynn effect: a meta-analysis, Psychol Bull source

      The Abstract reports 2.93 IQ points per decade for modern Stanford-Binet and Wechsler tests, supporting the approximate three-point figure for that subset. But its overall estimate across 285 studies since 1951 is 2.31 points per decade; the paper does not establish a three-point average across the past century. (LLM, conf=0.88)

      Experimental — accuracy not yet measured

  • May combine two claims.

    Premisecitation

    Height is a good proxy for population nutritional status at the cohort level, but due to high heritability it is not precise at the individual level, and there may be limited variance in critical nutritional factors in the Danish population.

    “For example, height may be a good proxy measure for the population’s nutritional status at the cohort level, but because of its high heritability, it is not precise at the individual level [20] and there may be limited variance in critical nutritional factors in the Danish population.”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites [20] JM Perkins, 2016, Adult height, nutrition, and population health, Nutr Rev source

      Split into 2 atoms — 0 confirmed, 0 partially supported, 0 disputed, 0 misattributed, 2 unverifiable. Expand the claim to see per-atom verdicts.

      Experimental — accuracy not yet measured

  • Classified as probably not a substantial claim.

    Premisecitation

    Height has been found to be a useful indicator of variation in cumulative net nutrition, particularly the intake of high-quality proteins.

    “Height (in centimetres) was used as a proxy for the sample’s nutritional status as it has been found to be a useful indicator of variation in cumulative net nutrition, particularly the intake of high-quality proteins [20].”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites [20] JM Perkins, 2016, Adult height, nutrition, and population health, Nutr Rev source

      The indexed Abstract calls adult height a useful marker of variation in cumulative net nutrition, supporting its use as a nutritional proxy. It does not specifically link height to intake of high-quality proteins, so that part of the claim is unsupported. (LLM, conf=0.85)

      Experimental — accuracy not yet measured

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    A previous study also observed a notable increase in Danish men's body height among those born between 1940 and 1980.

    “This observation is in line with a previous study, which also observed a notable increase in Danish men’s body height among those born between 1940 and 1980 [26].”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites [26] NCD Risk Factor Collaboration (NCD-RisC), 2016, A century of trends in adult human height, eLife source

      The Results report gains in European men’s height and state that Danish men born in 1996 averaged over 181 cm. They do not report a height increase specifically among Danish men born between 1940 and 1980, so the cited paper does not establish the claimed cohort trend. (LLM, conf=0.76)

      Experimental — accuracy not yet measured

  • May combine two claims.

    Interpretationcitation

    The beneficial influence of nutrition may have reached its limit during the study period, which would explain the decreasing association between height and intelligence and the decreasing intelligence test score gains over time.

    “However, the beneficial influence of nutrition may, by and large, have reached its limit during the study period, which would also explain the decreasing association between height and intelligence [28] and the decreasing intelligence test score gains over time.”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites [28] TW Teasdale, 1989, Fall in association of height with intelligence and educational level, BMJ source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The positive influence of education on intelligence test scores may be due to mechanisms such as teaching information relevant to intelligence tests, teaching thinking styles, and inculcating concentration and self-control.

    “Generally, the positive influence of education on intelligence test scores may be due to various mechanisms, such as the teaching of information directly relevant to intelligence tests, the teaching of thinking styles, and the inculcating of concentration and self-control that may improve test performance [30, 31].”
    Quote in the paperNot a substantial claim1 of 2 sources checked
    • cites [30] SJ Ceci, 1991, How much does schooling influence general intelligence and its cognitive components? A reassessment of the evidence, Developmental Psychology source
    • cites [31] SJ Ritchie, 2018, How much does education improve intelligence? A meta-analysis, Psychol Sci source

  • May combine two claims.

    Findingown data

    The cohort-level correlations between potential explanatory factors and intelligence test scores were r=-0.75 for family size, r=0.97 for height, and r=0.99 for education.

    “The cohort-level correlations between the potential explanatory factors and intelligence test scores were rfamily size: -0.75, rheight: 0.97, and reducation: 0.99.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingnone given

    The Danish population's fertility rate first increased from 2.2 children per woman in 1940 to 3.0 in 1946, after which it declined to 1.5 in 1980.

    “More specifically, the Danish population’s fertility rate first increased from 2.2 children per woman in 1940 to 3.0 children per woman in 1946, after which it declined to 1.5 children per woman in 1980.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    The estimated average increase in IQ was 3.8 points for the birth cohorts of 1940-49, 2.5 for 1950-59, 1.3 for 1960-69, and 0.3 for 1970-79.

    “More specifically, we estimated an average increase of 3.8 IQ points for the birth cohorts of 1940–49, 2.5 IQ points for the birth cohorts of 1950–59, 1.3 IQ points for the birth cohorts of 1960–69, and 0.3 IQ points for the birth cohorts of 1970–79.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    The IQ variability declined strongly across birth cohorts; the birth cohort of 1940 had a standard deviation of 15, whereas the birth cohort of 1980 had a standard deviation of 12.2.

    “Moreover, the IQ variability declined strongly across birth cohorts. Thus, the birth cohort of 1940 had a standard deviation of 15, whereas the birth cohort of 1980 had a standard deviation of 12.2.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    The secular increase in intelligence test scores coincided with changes in the three potential explanatory factors, manifesting in large amounts of explained variance in statistical models.

    “In other words, the secular increase in intelligence test scores coincided with changes in the three potential explanatory factors, manifesting itself in the statistical models’ large amounts of explained variance.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    We observed simultaneous increases in body height and intelligence.

    “Improved prenatal and postnatal nutrition may also have contributed to the secular increase in intelligence test scores as nutrition, particularly high-quality protein, influences the makeup of the human body, such as body height, head circumference, and intelligence [20, 23–25] and we observed simultaneous increases in body height and intelligence.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The estimated average decrease in IQ was 0.6 points for the birth cohorts of 1980-89 and 1.3 points for 1990-99.

    “Hereafter, we estimated an average decrease of 0.6 IQ points for the birth cohorts of 1980–89 and 1.3 IQ points for the birth cohorts of 1990–99.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    The estimated mean IQ increased from 100 (SD 15) for those born in 1940 to 108.9 (SD 12.2) for those born in 1980, since when it has been decreasing.

    “Interpolating the mean for those birth cohorts that did not have total intelligence test scores available for all tested conscripts suggested that the IQ mean increased from 100 (SD: 15) for those born in 1940 to 108.9 (SD: 12.2) for those born in 1980 since when it has been decreasing.”
    • Also stated in: § Abstract · hedged
      “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.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    Among birth cohorts of 1940 to 1980, the population's mean family size decreased while height (a proxy for nutritional status) and educational level increased.

    “Focusing on the birth cohorts of 1940 to 1980, for whom a secular increase in intelligence test scores was estimated, we observed that the population’s mean family size simultaneously decreased, whereas height as a proxy for the population’s nutritional status and educational level increased.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The empirical data show a sudden drop of 1.0 IQ points from those born in 1991 to 1992, coinciding with the change in test format.

    “a sudden drop of 1.0 IQ points from those born in 1991 to 1992 (coinciding with the change in the format of the intelligence test from a paper-and-pencil booklet to a computer-administered format)”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.
    • Rests on an assumption: The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

  • May combine two claims.

    Findingown data

    The estimated mean IQ score increased from 100 (SD 15) among men born in 1940 to 108.9 (SD 12.2) among men born in 1980, whereafter it decreased.

    “This Danish population-based study found that the estimated mean IQ score increased from a baseline set to 100 (SD: 15) among men born in 1940 to 108.9 (SD: 12.2) among men born in 1980, whereafter it decreased.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    The secular trend in intelligence test scores has not been linear but has been accelerating and decelerating and decreasing more recently.

    “However, the trend has not been linear but rather seems to have been accelerating and decelerating and finally decreasing more recently [4].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source

  • Classified as probably not a substantial claim.

    Premisecitation

    Populations' mean intelligence test scores have been rising since the beginning of the twentieth century.

    “Populations’ mean intelligence test scores have been rising since the beginning of the twentieth century.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [1] JR Flynn, 1984, The mean IQ of Americans: massive gains 1932 to 1978, Psychological Bulletin source
    • cites [2] JR Flynn, 1987, Massive IQ gains in 14 nations: what IQ tests really measure, Psychological Bulletin source

  • May combine two claims.

    Interpretationreasoning

    A larger proportion of firstborn children will contribute positively to the intellectual environment of everyone in the birth cohort, potentially increasing the birth cohort's mean intelligence test score further.

    “However, a larger proportion of firstborn children will contribute positively to the intellectual environment of everyone in the birth cohort and for this reason, the birth cohort’s mean intelligence test score may increase further.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The increases in body height and intelligence occurred at a time when both the quantity and quality of the Danish population's nutrition, particularly meat consumption, increased markedly due to improvements in living standards.

    “Moreover, the increases in body height and intelligence occurred at a time during which both the quantity and quality of the Danish population’s nutrition, particularly its meat consumption, increased markedly as a consequence of considerable improvements in living standards [27].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [27] A Biltoft-Jensen, 2016

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    A data synthesis reports a reversed Flynn effect in 13 countries in Africa, Europe, and North and South America, suggesting the reversed secular trend has strengthened in more recent years.

    “and a data synthesis based on the aforementioned systematic review and another systematic review of a subset of secular losses likewise reports a reversed Flynn effect in 13 countries in Africa, Europe, and North and South America (study mid-years spanning from 1920.5 through 2007.5), suggesting that the reversed secular trend has strengthened in more recent years [7].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [7] MA Woodley of Menie, 2018, What causes the anti-Flynn effect? A data synthesis and analysis of predictors, Evolutionary Behavioral Sciences source

  • May combine two claims.

    Study design and conductown data

    In sensitivity analyses solely based on data from birth cohorts 1940-1958, the findings were generally consistent, except that the cohort-level correlation between family size and mean intelligence test scores was positive and family size explained a little less of the variance.

    “The findings were generally consistent, except that the cohort-level correlation between family size and mean intelligence test scores was positive and family size explained a little less of the variance in mean intelligence test scores.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    The inconsistent findings on family size and intelligence may be due to contextual differences or the meta-analysis' crude analysis averaging across time periods with both increasing and decreasing family sizes.

    “The inconsistent findings may be due to contextual differences in the influence of family size or due to the meta-analysis’ crude analysis estimating an average influence of family size across time periods with both increasing and decreasing family sizes.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    The estimated curvilinear development in mean intelligence test scores of Danish male birth cohorts born between 1940 and 2000 is supported by previous Danish studies based on somewhat selected samples and partly by a global meta-analysis reporting decreasing intelligence test score gains in more recent decades.

    “The estimated curvilinear development in mean intelligence test scores of the Danish male birth cohorts born between 1940 and 2000 is supported by previous Danish studies based on somewhat selected samples of our study population and also partly by a global meta-analysis of studies covering an entire century reporting decreasing intelligence test score gains in more recent decades [4, 9–14].”
    Quote in the paperNot a substantial claim0 of 7 sources checked
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source
    • cites [9] ER Hegelund, 2021, The secular trend of intelligence test scores in the present century: the Danish experience, Intelligence source
    • cites [10] TW Teasdale, 1987, National secular trends in intelligence and education: a twenty-year cross-sectional study, Nature source
    • cites [11] TW Teasdale, 1989, Continuing secular increases in intellgence and a stable prevalence of high intelligence levels, Intelligence source
    • cites [12] TW Teasdale, 2000, Forty-year secular trends in cognitive abilties, Intelligence source
    • cites [13] TW Teasdale, 2005, A long-term rise and recent decline in intelligence test performance: the Flynn effect in reverse, Personality and Individual Differences source
    • cites [14] TW Teasdale, 2008, Secular declines in cognitive test scores: a reversal of the Flynn Effect, Intelligence source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Danish studies have suggested that the observed secular trend of intelligence test scores has mirrored the country's educational changes.

    “Danish studies have suggested that the observed secular trend of intelligence test scores has mirrored the country’s educational changes [10–12].”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites [10] TW Teasdale, 1987, National secular trends in intelligence and education: a twenty-year cross-sectional study, Nature source
    • cites [11] TW Teasdale, 1989, Continuing secular increases in intellgence and a stable prevalence of high intelligence levels, Intelligence source
    • cites [12] TW Teasdale, 2000, Forty-year secular trends in cognitive abilties, Intelligence source

  • May combine two claims.

    Premisenone given

    Previous Danish studies are mainly based on selected samples, samples from a few calendar years, or samples with only crudely categorized intelligence test scores.

    “However, the previous studies are mainly based on selected samples (e.g. men with specific birthdates and places of residence), samples from a few calendar years, or samples with only crudely categorized intelligence test scores available.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    Education is much more strongly related to intelligence than family size and height, though this may partly reflect reciprocal causation because intelligence influences educational attainment.

    “These correlations suggest that education is much more strongly related to intelligence than the two other factors, but this may to some extent reflect reciprocal causation as intelligence influences the individual’s educational attainment.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    A meta-analysis found that fertility rate was positively related to intelligence test score gains.

    “Thus, a meta-analysis found that fertility rate was positively related to intelligence test score gains [4].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source

  • May combine two claims.

    Premisecitation

    The most recent study included the entire Danish population of men examined between 2006 and 2019 (birth cohorts 1987-2000) and explored possible explanations of the secular decline.

    “Only the most recent study has had the possibility to include the entire Danish population of men examined between 2006 and 2019 (closely corresponding to the birth cohorts of 1987–2000) and explore possible explanations of the secular decline in intelligence test scores [9].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [9] ER Hegelund, 2021, The secular trend of intelligence test scores in the present century: the Danish experience, Intelligence source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationcitation

    The curvilinear development seems to be virtually identical with changes in spatial ability observed in German-speaking countries during 1977 to 2014.

    “Moreover, the curvilinear development seems to be virtually identical with changes in spatial ability observed in German-speaking countries during the period of 1977 to 2014 [21].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [21] J Pietschnig, 2015, A reversal of the Flynn effect for spatial perception in German-speaking countries: evidence from a cross-temporal IRT-based meta-analysis (1977–2014), Intelligence source

  • May combine two claims.

    Premisecitation

    Suboptimal prenatal and postnatal nutrition reduces body height, head circumference, and intelligence, while improvements in nutrition positively influence the human body, including brain development and functioning.

    “While suboptimal prenatal and postnatal nutrition has been found to reduce body height, head circumference, and intelligence, improvements in nutrition on the other hand have a positive influence on the human body, including the development and the functioning of the brain [23, 24].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [23] R. Lynn, 1989, A nutrition theory of the secular increases in intelligence; positive correlations between height, head size and IQ, British Journal of Educational Psychology source
    • cites [24] R. Lynn, 1990, The role of nutrition in secular increases in intelligence, Personality and Individual Differences source

  • May combine two claims.

    Study design and conductown data

    The two linear regression models explained 96.8% and 99.0% of the variance in the birth cohorts' IQ mean and SD, respectively.

    “The two models explained 96.8% and 99.0% of the variance in the birth cohorts’ IQ mean and SD, respectively”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    Previous Danish studies based on young male conscripts suggested that intelligence test scores increased with a decelerating rate between 1959 and 1998 (birth cohorts 1940-1979), since then a slight fall and subsequent stagnation have been observed.

    “Previous Danish studies based on young male conscripts have suggested that intelligence test scores have increased with a decelerating rate between 1959 and 1998 (i.e. between the birth cohorts of 1940 and 1979), since then a slight fall and a subsequent stagnation have been observed [9–14].”
    Quote in the paperSubstantial claim0 of 6 sources checked
    • cites [9] ER Hegelund, 2021, The secular trend of intelligence test scores in the present century: the Danish experience, Intelligence source
    • cites [10] TW Teasdale, 1987, National secular trends in intelligence and education: a twenty-year cross-sectional study, Nature source
    • cites [11] TW Teasdale, 1989, Continuing secular increases in intellgence and a stable prevalence of high intelligence levels, Intelligence source
    • cites [12] TW Teasdale, 2000, Forty-year secular trends in cognitive abilties, Intelligence source
    • cites [13] TW Teasdale, 2005, A long-term rise and recent decline in intelligence test performance: the Flynn effect in reverse, Personality and Individual Differences source
    • cites [14] TW Teasdale, 2008, Secular declines in cognitive test scores: a reversal of the Flynn Effect, Intelligence source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The BPP is essentially a test of fluid intelligence.

    “However, the intelligence test is essentially a test of fluid intelligence [18, 19].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [18] P Hartmann, 2004, A test of Spearman’s “Law of Diminishing Returns” in two large samples of Danish military draftees, Intelligence source
    • cites [19] P Hartmann, 2005, Spearman’s “Law of Diminishing Returns” and the role of test reliability investigated in a large sample of Danish military draftees, Personality and Individual Differences source

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Decreasing family sizes may lead to increasing mean intelligence test scores due to birth order effects, as intelligence test scores decrease with increasing birth order because parents have less time and resources per child.

    “As intelligence test scores have been found to decrease with increasing birth order because parents have less time and resources to spend on each child, populations’ decreasing family sizes may lead to increasing mean intelligence test scores simply due to the larger proportion of firstborn children.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [22] RB Zajonc, 1997, Birth order: Reconciling conflicting effects, American Psychologist source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The BPP total score correlates highly with Raven's Progressive Matrices.

    “and Raven’s Progressive Matrices [17].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [17] TW Teasdale, 2011, The reliability and validity of the Danish draft board cognitive ability test: Børge Prien’s Prøve, Scand J Psychol source

  • Classified as probably not a substantial claim.

    Study design and conductreasoning

    This correlational study cannot provide evidence of causal influences of the potential explanatory factors.

    “However, it is important to note that this correlational study in itself cannot provide evidence of the causal influences of the potential explanatory factors”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing), S4 (based). May combine two claims.

    Premisenone given

    Previous studies have suggested that improvements in the population's educational level and larger intelligence test score gains among the least able resulting in less IQ variability are important contributors to the last century's secular increase in intelligence test scores.

    “Nonetheless, the previous studies have suggested that improvements in the population’s educational level and larger intelligence test score gains among the least able resulting in less IQ variability are important contributors to the last century’s secular increase in intelligence test scores, although other possible explanations remain unexplored.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The most recent study found that changes in test format (from paper-and-pencil to computerized) and increasing sample selection are the most important contributors to the observed secular decline.

    “but that changes in test format (from paper-and-pencil to computerized) and increasing sample selection are the most important contributors.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [9] ER Hegelund, 2021, The secular trend of intelligence test scores in the present century: the Danish experience, Intelligence source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Current evidence points towards components of the life history speed theory, such as improvements in education and nutrition and a reduction in the prevalence of infectious diseases and other pathogens, as the most important contributing factors to the Flynn effect.

    “However, current evidence points towards components of the life history speed theory [8], such as improvements in education and nutrition and a reduction in the prevalence of infectious diseases and other pathogens, as being the most important contributing factors [4].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [8] MA Woodley, 2012, A life history model of the Lynn–Flynn effect, Personality and Individual Differences source
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    Since only total intelligence test scores were available, it was not possible to evaluate whether the intelligence construct changed over time.

    “However, since only total intelligence test scores were available, it was not possible to evaluate whether our intelligence construct changed over time.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    The influence of education on the secular trend in intelligence test scores is supported by a wide range of evidence, including findings of a substantial global increase in crystallized intelligence and stronger intelligence test score gains among adults than children.

    “The influence of education on the secular trend in intelligence test scores is supported by a wide range of evidence, including findings of a substantial global increase in crystallized intelligence and stronger intelligence test score gains among adults than children [4].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [4] J Pietschnig, 2015, One century of global IQ gains: a formal meta-analysis of the Flynn effect (1909–2013), Perspect Psychol Sci source

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Study design and conductreasoning

    The use of the same test makes obsolescence of test items possible, but this is less of a concern because the test items involve abstract reasoning, which is less susceptible to obsolescence.

    “Although the use of the same test also makes obsolescence of test items possible, this is less of a concern, given that the test items involve abstract reasoning, which is less susceptible to obsolescence.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    It has been shown that the secular increase was strongest among those with low intelligence test scores and that this coincided with an increase in the population's minimum educational level and increasing resources allocated to remedial and supplementary education.

    “Thus, it has been shown that the secular increase was strongest among those with low intelligence test scores and that this coincided with an increase in the population’s minimum educational level and increasing resources being allocated to remedial and supplementary education.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [10] TW Teasdale, 1987, National secular trends in intelligence and education: a twenty-year cross-sectional study, Nature source
    • cites [11] TW Teasdale, 1989, Continuing secular increases in intellgence and a stable prevalence of high intelligence levels, Intelligence source
    • cites [12] TW Teasdale, 2000, Forty-year secular trends in cognitive abilties, Intelligence source

  • May combine two claims.

    Premisecitation

    We have previously shown that the positive influence on intelligence depends on the educational level and starts to decline after upper-secondary school.

    “Moreover, we have previously shown that the positive influence on intelligence depends on the educational level and that it starts to decline after upper-secondary school [29], which supports our observation of smaller increases in the population’s mean intelligence test score over time.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [29] ER Hegelund, 2020, The influence of educational attainment on intelligence, Intelligence source

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    The explanatory factors contributing to the secular trend may be more generalizable, although the magnitude of their impact may be limited to societies similar to Danish society during the study period.

    “However, the explanatory factors contributing to the secular trend may be more generalizable although the magnitude of their impact likewise may be limited to societies similar to the Danish society during the study period.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Study design and conductown data

    The study's major strength is its large study population (>1 million Danish men born 1940-1958 and 1987-2000 appearing before a draft board until 2020), enabling analysis of the secular trend over sixty years.

    “The major strength of the present study is its large study population, including >1 million Danish men born in 1940–1958 and 1987–2000 and appearing before a draft board until 2020, which has made it possible to analyse the secular trend of intelligence test scores in the same population over sixty years.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Study design and conductcitation

    There has been a rising proportion of conscripts exempted from draft board examinations over time, and a previous study suggested this may have artificially inflated the population mean among the youngest birth cohorts because exempted individuals generally seem worse off on characteristics correlated with intelligence.

    “Moreover, there has been a rising proportion of conscripts getting exempted from the draft board examinations over time and a previous study has suggested that this may, artificially, have inflated the population mean among the youngest birth cohorts as the group of exempted individuals generally seems to be worse off with regard to various characteristics correlated with intelligence [9].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [9] ER Hegelund, 2021, The secular trend of intelligence test scores in the present century: the Danish experience, Intelligence source

  • May combine two claims.

    Study design and conductown data

    Only the birth cohorts born in 1940-1958 and 1987-2000 had total intelligence test scores available for all tested conscripts, and their distributions were used to interpolate the secular trend during the entire study period.

    “Another limitation is that only the birth cohorts born in 1940–1958 and 1987–2000 had total intelligence test scores available for all tested conscripts and, therefore, their intelligence test score distributions were used to interpolate the secular trend during the entire study period.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    There is no evidence of major sex differences in general intelligence test scores.

    “Finally, it should be mentioned that there is no evidence of major sex differences in general intelligence test scores [35]”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [35] IJ Deary, 2012, Intelligence, Annual Review of Psychology source

  • May combine two claims.

    Study design and conductown data

    The investigation of possible contributors may have been influenced by the specific birth cohorts with available intelligence test scores; sensitivity analysis suggested the influence of family size differed when considering only 1940-1958 versus all birth cohorts born until 1980 with available scores.

    “Also, the investigation of the possible contributors to the secular increase in intelligence test scores may have been influenced by the specific birth cohorts with available intelligence test scores; for instance, the sensitivity analysis suggested that the influence of family size differed when considering only the birth cohorts of 1940–1958 versus all birth cohorts born until 1980 with”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    The reduced variability may be explained by improved nutrition primarily benefiting malnourished individuals and educational changes mainly improving the abilities of the least able.

    “This reduced variability may, as suggested, both be explained by improved nutrition primarily benefiting malnourished individuals [32, 33] and educational changes mainly improving the abilities of the least able [34].”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [32] R Colom, 2005, The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: Supporting evidence for the nutrition hypothesis, Intelligence source
    • cites [33] RE Nisbett, 2012, Intelligence: New findings and theoretical developments, American Psychologist source
    • cites [34] H Rindermann, 2013, Ability rise in NAEP and narrowing ethnic gaps?, Intelligence source

  • May combine two claims.

    Interpretationreasoning

    Our findings support the hypothesis that both nutrition and education may be important contributing factors to the secular increase in mean intelligence test scores and the decreasing variance.

    “Thus, our findings support the hypothesis that both nutrition and education may be important contributing factors to the secular increase in mean intelligence test scores and the decreasing variance.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    Other strengths include use of information collected at the draft board examination, including educational level and height measured on-site.

    “Other strengths include the use of information collected at the draft board examination, including, inter alia, educational level and height measured on-site.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Study design and conductown data

    The regression model used to interpolate missing birth cohort values explained a large proportion of the variance in mean intelligence test scores, but the estimated mean differed from the empirical mean observed during the last birth decade.

    “The regression model used to interpolate the missing birth cohort values explained a large proportion of the variance in mean intelligence test scores, but as previously written the estimated mean differed from the empirical mean observed during the last birth decade.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    It was not possible to evaluate whether the secular trend was homogeneous across subtests or depended on subtests' g-loadings.

    “Likewise, it was not possible to evaluate whether the secular trend of intelligence test scores was homogeneous across subtests or if the trend depended on the subtests’ g-loadings which might have given us hints about which factors contributed to the observed secular increase.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    In a Danish context, both explanations seem reasonable since the decreasing intelligence test score variance coincided with increasing high-quality protein intake, an increase in the population's minimum educational level, and increasing resources allocated to remedial and supplementary education.

    “In a Danish context, both of these explanations seem reasonable since the decreasing intelligence test score variance coincided with an increasing high-quality protein intake, as well as an increase in the population’s minimum educational level and increasing resources being allocated to remedial and supplementary education.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Study design and conductnone given

    The use of the same well-validated intelligence test is a strength.

    “The use of the same well-validated intelligence test can likewise be considered a strength.”
    Quote in the paperSubstantial claim
59 of 59

  • The paper relies on this without support.

    The BPP intelligence test scores are comparable across the different conscription databases and the 2010 change from paper-and-pencil to computer-administered format.

    “The items in the subtests have remained unchanged to the present day, although their presentation changed in 2010 from a paper-and-pencil booklet to a computer-administered format.”
    “Information on intelligence test scores was available from four conscription databases covering various birth cohorts: the Danish Conscription Database (~1940–1958), the Danish National Archive’s database (~1969–1986), the Danish Defence Personnel Organisation’s database (~1976–1986), and the Conscription Registry (~1987-).”

  • The paper relies on this without support.

    The draft board sample is representative of the Danish male birth cohorts for estimating secular trends in intelligence.

    “About half of those declared ineligible never appear before a draft board as they are exempted from military service due to documentation of health issues forwarded in advance to the draft board.”
    “However, this number has increased substantially in the last decade from 13% of the men born in 1987 to around 20% of the men in the youngest birth cohorts.”

  • The authors state this as an assumption.

    If an explanatory factor has a substantial causal influence on the observed secular increase in intelligence test scores, it must show a high correlation with intelligence at the cohort level.

    “We assume that if an explanatory factor has a substantial causal influence on the observed secular increase in intelligence test scores, it must show a high correlation with intelligence at the cohort level.”

  • The paper relies on this without support.

    Many factors related to the population's life circumstances change over time, and changes in some factors may correlate with changes in birth cohorts' mean intelligence test scores even though the two phenomena are completely unrelated.

    “However, many factors related to the population’s life circumstances change over time and, presumably, changes in some factors may correlate with changes in the birth cohorts’ mean intelligence test scores even though the two phenomena are completely unrelated.”
4 of 4

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Fertility rate is given in incompatible units

    Minor

    The methods define the cohort-level fertility rate as live births per 1,000 women, but the results express that rate as children per woman. Those are different measures, with no conversion explained.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 1

  1. A Biltoft-Jensen, 2016, Køds rolle i kosten

Cited by

Sorted by citations

Related

Authors
Hegelund, E. R., Teasdale, T. W., Okholm, G. T., Osler, M., Sørensen, T. I. A., Christensen, K., & Mortensen, E. L.
Affiliations
  • University of Copenhagen (DK)
  • University of Southern Denmark (DK)
Corresponding author
Hegelund, Emilie R.
ORCID
Venue
PLOS ONE
Publisher
Public Library of Science (PLoS)
Published
2021-12-09
Volume
16
Issue
12
Pages
e0261117
ISSN
1932-6203 (ISSN-L)
DOI
10.1371/journal.pone.0261117
PMC
PMC8659667
OpenAlex
W4200601319
Semantic Scholar
25add018cf078ae045afe3f4cf0d870979c79548
Cited by
12
References
36
Citations by year
2026: 1 · 2025: 3 · 2024: 3 · 2023: 3 · 2022: 2
FWCI
1.93
Type
journal article
Language
en
Open access
Licence
Locations
  • PLoS ONE (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Research at the University of Copenhagen (University of Copenhagen) (publishedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • University of Southern Denmark Research Portal (University of Southern Denmark) (submittedVersion, cc-by)
Topics
  • Social Sciences › Psychology › Experimental and Cognitive Psychology › Cognitive Abilities and Testing
  • Social Sciences › Sociology and Political Science › Intergenerational and Educational Inequality Studies
  • Social Sciences › Economics, Econometrics and Finance › Economics and Econometrics › Historical Economic and Social Studies
Keywords
secular trends, intelligence test scores, Flynn effect, birth cohort, family size, nutrition, height, Danish men
MeSH terms
Birth Cohort, Adult, Body Height, Cognition, Denmark, Educational Status, Female, Humans, Intelligence, Intelligence Tests, Male, Time Factors, Cohort Studies, Young Adult

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-11-03.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-11-03.

Cited sources are checked by gpt-6-sol.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

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