Flynn effect: Average IQ scores increase each decade for most of the twentieth century. Intelligence tests capture not just the innate cognitive ability but also the environmental changes, including societal changes that allow and reward abstract thinking.
If you take the same IQ test your grandfather once faced, there's a high chance that you will outscore him without extra study.
James R. Flynn first discovered this phenomenon: both fluid and crystallized intelligence test scores that were measured substantially and sustainedly increased over the 20th century. He saw average scores steadily climbing about three points every ten years from the 1930s to the 1970s. And this rise showed up almost everywhere: in the United States, the United Kingdom, and many other industrial nations.
From Mohamed Nagdy - https://web.archive.org/web/20220306071619/https://ourworldindata.org/intelligence
Why? Although the exact causes of the Flynn effect remain uncertain, several environmental factors are possible candidates: Education became more accessible, nutrition improved, and everyday life got more complex.
Nowadays, daily life is filled with puzzles, screens, and symbols, which naturally exercise people's abilities in abstract thinking and problem-solving. All those forces boosted fluid reasoning and stored knowledge, the very skills IQ tests measure. Therefore, rising IQ scores are not just adaptive to life in the modern world, but are also enabled by it.
Modern society also affords the "luxury" of being able to think about the world in abstract terms in the first place. Its economy further rewards individuals who can reason more in abstract terms.
Therefore, Flynn himself questioned whether the rise in IQ scores really meant that people were getting more intelligent. Perhaps, we are not "smarter" but are more privileged. This privilege comes from living in a society that allows and rewards abstract thinking.