According to wikipedia:
The Flynn effect is the increase of the average intelligence quotient (IQ) test scores over generations (IQ gains over time). The effect has also been reported for other cognitions such as semantic and episodic memory.[1] The effect occurs in most parts of the world although at greatly varying rates.
The Flynn effect is named for James R. Flynn, who did much to document it and promote awareness of its implications. However, the first to invent the term was not Flynn himself but the authors of The Bell Curve.[2]
The effect increase has been continuous and approximately linear from the earliest years of testing to the present. "Test scores are certainly going up all over the world, but whether intelligence itself has risen remains controversial," psychologist Ulric Neisser wrote in an article during 1997 in The American Scientist.[3] The Flynn effect may have ended in some developed nations starting during the mid 1990s although other studies, such as Black Americans reduce the racial IQ gap: Evidence from standardization samples (Dickens, Flynn; 2006), still show gains between 1972 and 2002.
Why?
Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis[8]. Another proposition is greater familiarity with multiple-choice questions and experience with brain-teaser IQ problems.