James Flynn documented the average rise in IQ scores were over the 20th century. IQ tests are “renormed” to keep the mean score at 100. IQ test makers consistently renormed it every generation. Flynn reviewed the history of IQ tests, and noticed a trend. Every decade, the mean IQ drifted upwards. The test-makers saw the mean-IQ rise to ~105, so the made the test more difficult to renorm it to 100. The next decade, the IQs rose to ~105 on the more difficult tests, so they renormed it…
The effect was strongest on the Raven Progressive Matrices which tests abstract thinking. The Flynn effect is weakest in culturally biased tests. Literally, you are probably smarter than your grandparents and by a fair measure.
GeneExpression estimates total IQ gains and related evidence to support the Flynn effect. Human height has grown, brain mass and cranial capacity grew, and puberty occurs at earlier ages. This suggests that better nutrition is leading to greater mental and physical development.
Agricultural societies must have been more impoverished than we thought. Poor nutrition meant most people were short and dumb as mules, and humanity is only reaching it biological potential after the Industrial Revolution.