Biology
Intelligence rose for a century, then reversed
IQ tests are reset so the average score is always 100. For most of the last century each generation scored higher than the one before, about three points a decade, so tests kept being made harder. The gains were largest on abstract reasoning, not knowledge, and since roughly 1975 the trend has reversed in several countries.
Sources & further reading
Psychological Bulletin (American Psychological Association)The mean IQ of Americans: Massive gains 1932 to 1978The study showing American IQ scores rose about 14 points across the mid-20th century, so tests had to be reset.Psychological Bulletin (American Psychological Association)Massive IQ gains in 14 nations: What IQ tests really measureThe paper showing the gains happened across many countries and mostly on abstract reasoning, not on facts.Cambridge University PressWhat Is Intelligence? Beyond the Flynn EffectFlynn's book explaining the rise as a modern habit of thinking in abstract categories.National Bureau of Economic Research (NBER Working Paper No. 19233)The Cognitive Effects of Micronutrient Deficiency: Evidence from Salt Iodization in the United StatesThe study showing iodized salt raised IQ by about 15 points for the most iodine-deficient children.Proceedings of the National Academy of Sciences (PNAS)Flynn effect and its reversal are both environmentally causedThe study showing IQ has reversed in Norway since the 1970s, and the fall shows up between brothers, so it is environmental.Science (American Association for the Advancement of Science)Nutrition and Mental PerformanceThe study finding that starvation in the womb during a wartime famine did not lower the men's adult IQ.
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