Flynn effect
Some scientists believe these changes are very big. One of them is Ulric Neisser. In 1995 he was the head of a task force of the American Psychological Association, charged with writing a statement on where intelligence research was. He estimates that if American children of 1932 could take an IQ test normed in 1997 their average IQ would have been only about 80. In other words, half of the children in 1932 would be classified as having borderline mental retardation or worse in 1997. Looking at Ravens, Neisser estimates that if you extrapolate beyond the data, which shows a 21 point gain between 1952 and 1982, an even larger gain of 35 IQ points can be argued.  Arthur Jensen warns that extrapolating beyond the data leads to results such as an IQ of minus 1000 for Aristotle (even assuming he would have scored 200 in his day).