A long-term rise and recent decline in intelligence test performance: the Flynn effect in reverse

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].”