The Flynn Effect refers to the consistent rise in intelligence test scores over time. It shows that average intelligence has increased by about three IQ points per decade since the early 20th century. This is likely due to improvements in nutrition, education, and more mentally stimulating environments and lifestyles in modern societies. However, the Flynn Effect seems to have stopped or even reversed in some Western nations after the 1990s. Differences in cultures and environments can affect the type of intelligence that develops.