Can IQ change over time?

IQ scores show considerable stability across adulthood, but they can change. Education is associated with gains: a meta-analysis by Ritchie and Tucker-Drob estimated roughly one to five additional IQ points per extra year of schooling, depending on study design. Scores also shift with practice, health, age-related changes in different abilities and the norms used. Across whole populations, average scores rose substantially during the twentieth century.

Average measured IQ rose across many countries during the twentieth century, a pattern named after researcher James Flynn. Pietschnig and Voracek's meta-analysis of data from 1909 to 2013 estimated gains of about 0.28 IQ points per year for full-scale scores, roughly 2.8 points per decade, with larger gains for fluid reasoning (about 0.41 points per year) than for crystallized knowledge (about 0.21). They also found gains decreasing across domains in recent decades and noted data from Denmark and Finland suggesting a reversal. Explanations include improvements in education, nutrition, health and familiarity with abstract reasoning.

Stability and change coexist. People tend to keep a similar position relative to their peers, while absolute performance can shift for everyone. That's why norms are updated: without that, rising average performance would gradually inflate everyone's scores. For an individual, a change of a few points between tests is usually within measurement error and practice effects rather than evidence of real change.