The “Flynn effect” refers to the observed rise in IQ scores over time, resulting in norms obsolescence. In other words, as the 20th century progressed, IQ tests had to be recalibrated, because on average people did better on them.
And not just a little bit better. A lot better:
In the 1980s, social scientist James Flynn made a startling discovery: Real IQ scores had been going up, on average, three points every decade since the early 20th century. The existence of this increase had been masked by the fact that the test gets updated and renormed every generation or so, pushing the average score back to 100.
The implications of the eponymous “Flynn effect” are astonishing. A person of average intelligence today would have registered a full two standard deviations higher a century ago, giving him a “very superior” score of 130. We’re getting smarter. A lot smarter. (Source)
When looked at closely, the gains have not been in mathematics or vocabulary, but specifically on the portions of the test most dedicated to abstract reasoning.
Flynn discusses his findings in this TED Talk:
Flynn talks here about Taking the Hypothetical Seriously.