Flynn effect and its reversal are both environmentally caused

Area of Science:

• Psychology
• Cognitive Science
• Behavioral Genetics

Background:

• The Flynn effect describes the observed rise in population intelligence quotients (IQ) throughout the 20th century.
• Recent decades show a slowdown or reversal of the Flynn effect in numerous countries, prompting research into its causes.
• Distinguishing between proposed explanations requires understanding factors that operate within families versus those that vary between them.

Purpose of the Study:

• To investigate the causal factors behind the Flynn effect, its recent reversal, and its decline.
• To differentiate between explanations by examining within-family variations in cognitive ability scores.

Main Methods:

• Utilized administrative register data and cognitive ability scores from Norwegian military conscription records.
• Analyzed data from birth cohorts spanning three decades (1962-1991).
• Employed within-family analyses to control for shared genetic and environmental factors among siblings.

Main Results:

• The study successfully replicated the Flynn effect, its turning point, and subsequent decline using within-family variation.
• Observed changes in cognitive abilities were fully explained by factors that differ between siblings.
• No evidence supported hypotheses suggesting genes or between-family environmental factors as primary drivers of the decline.

Conclusions:

• The Flynn effect and its recent reversal are driven by factors that vary within families.
• The findings challenge prominent hypotheses that attribute the trend to genetic or broad environmental changes affecting all siblings equally.
• Future research should focus on intrafamilial factors to understand trends in population intelligence.