The Flynn effect describes the significant, sustained increase in both fluid and crystallized intelligence scores globally during the 20th century. Average IQ scores have risen continuously and linearly, with notable increases observed in multiple countries including the UK, Japan, and South Korea.

Various reasons have been proposed for the Flynn effect, such as improved education and health, but there is also skepticism about the broader implications of these gains. Recent studies indicate a potential reverse Flynn effect, with declining IQ scores since the 1990s in several countries, despite high academic performance in international assessments.

The term "Flynn effect" was named after James Flynn by Richard Herrnstein and Charles Murray, although Flynn himself credited earlier researcher Read D. Tuddenham for initially identifying significant IQ score increases.