Flynn effect

The Flynn effect is the observed rise of raw IQ scores by roughly 3 points per decade across the 20th century. Tests are re-normed to keep the mean at 100, so the rise is invisible in reported scores but real in raw data.

The likely causes include improved nutrition, schooling, exposure to abstract reasoning (via media and formal tests) and smaller family sizes. In recent decades, the effect has slowed or reversed in some developed countries.