The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points. When IQ tests are revised, they are again standardized using a new sample of test-takers, usually born more recently than the first; the average result is set to 100. When the new test subjects take the older tests, in almost every case their average scores are significantly above 100.

Test score increases have been continuous and approximately linear from the earliest years of testing to the present. For example, a study published in the year 2009 found that British children's average scores on the Raven's Progressive Matrices test rose by 14 IQ points from 1942 to 2008. Similar gains have been observed in many other countries in which IQ testing has long been widely used, including other Western European countries, as well as Japan and South Korea.

There are numerous proposed explanations of the Flynn effect, such as the rise in efficiency of education, along with skepticism concerning its implications. Similar improvements have been reported for semantic and episodic memory. Some research suggests that there may be an ongoing reversed Flynn effect (i.e., a decline in IQ scores) in Norway, Denmark, Australia, Britain, the Netherlands, Sweden, Finland, and German-speaking countries. This is said to have started in the 1990s and to be occurring despite the average performance of 15-year olds in those same countries ranking above the international average on the OECD Programme for International Student Assessment in reading, mathematics, and science in 2000, 2003, 2006, 2009, 2012, 2015, and 2018. In certain cases, this apparent reversal may be due to cultural changes which render parts of intelligence tests obsolete. Meta-analyses indicate that, overall, the Flynn effect continues, either at the same rate, or at a slower rate in developed countries.