The secular trend of intelligence test scores in the present century: the Danish experience

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.”

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].”

In a previous study, the authors found that changes in sample composition during the last decade may have inflated the mean intelligence test scores as the proportion of exempted increased and this group generally seemed to be worse off with regard to characteristics that were all correlated with intelligence.

“However, in a previous study, we found that changes in sample composition during this decade may have inflated the mean intelligence test scores as the proportion of exempted increased and this group generally seemed to be worse off with regard to characteristics that were all correlated with intelligence [9].”

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].”

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].”

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].”