The Flynn Effect Explained: Why IQ Scores Rose for a Century
Raw IQ scores rose about three points per decade through most of the 20th century. What caused it, and why the trend has reversed in some countries.
If you scored 100 on an IQ test normed in 1950 and could somehow take the 2000 version, you would score lower, not because you got worse, but because the average person in 2000 answered more questions correctly. That is the Flynn effect: raw scores on intelligence tests rose steadily through most of the twentieth century. It is one of the most surprising findings in psychology, and it has no single settled explanation.
What James Flynn found
In the 1980s the political scientist James Flynn compared the results of IQ tests given to American military recruits and test standardisation samples across decades. He found that average performance rose by roughly 3 IQ points per decade, and similar gains in dozens of countries followed. Richard Lynn had noticed related patterns earlier, and the effect is sometimes called the Flynn-Lynn effect.
The gains were largest on tests of fluid reasoning such as Raven's Progressive Matrices, the type of puzzle used in our IQ test, and smaller on vocabulary and general knowledge. Over a century, three points per decade adds up to thirty points: that is, if the trend were taken literally, an average person from 1900 would score near 70 against today's norms.
Why re-norming matters
Because scores drift upwards, test publishers must periodically re-standardise their tests so that the average stays at 100. This is why the same raw performance gives a lower IQ on a newer edition of a test. In practice it also means that an old test, scored against old norms, overestimates an IQ. This is one reason professionals avoid using outdated test versions, and one reason that a test with no recent norms, like many online quizzes, should be treated cautiously.
What might explain it?
Nobody believes human genes changed that fast, so the explanation lies in environment. Candidates that researchers have proposed include:
- More and better schooling. Years of education rose sharply, and schooling trains the abstract, rule-based thinking that matrix tests reward.
- Better nutrition and health. Lower infectious disease burden and better early-life nutrition support brain development. Height rose over the same period.
- More complex surroundings. Modern life exposes people to symbols, maps, schedules, and visual media. Flynn suggested people increasingly learned to apply "scientific spectacles", classifying and reasoning abstractly.
- Smaller families and test familiarity. Both have been suggested, though their effects appear modest.
It is likely that several factors contributed, and different ones may matter in different countries and eras.
The effect is stalling or reversing in some places
Since roughly the 1990s, several wealthy countries have shown a flattening or even a decline. Studies of Norwegian conscripts, for instance, found that scores peaked among men born in the mid-1970s and declined afterwards. Bratsberg and Rogeberg (2018) compared brothers within families and found the decline appeared within families, which points to environmental causes rather than changes in who was having children. Similar patterns have been reported in Denmark, Finland and some other countries, though not everywhere. In developing countries the effect often continues, consistent with ongoing gains in nutrition and schooling.
The reversal is also not fully explained. Suggested contributors include changes in education, reading habits, and the way young people spend time, but the evidence for any one cause is limited.
Does it mean we got smarter?
That is contested. Flynn himself argued that the gains reflect a shift in the kinds of thinking people practise, towards abstract classification, rather than a general rise in brain power. If people had really become far more intelligent in every respect, we would expect a flood of geniuses and a collapse in the numbers of people with intellectual disability; neither occurred. Test scores rose; the wider meaning of that rise is still debated.
What this means for you
- IQ scores are always relative to a reference population and a date. A score is not a constant of nature.
- Comparing the IQs of people from different generations or countries is hard, because norms differ.
- Environment, especially education and early health, seems to shape cognitive test performance more than the headline "IQ is genetic" picture suggests, even though heritability within a given population can still be substantial.
For an introduction to what the scores mean in the first place, see what is a good IQ score?.
Sources
- Flynn, J. R. (1987). Massive IQ gains in 14 nations: What IQ tests really measure. Psychological Bulletin, 101(2), 171–191.
- Flynn, J. R. (2007). What Is Intelligence? Beyond the Flynn Effect. Cambridge University Press.
- Pietschnig, J., & Voracek, M. (2015). One century of global IQ gains: A formal meta-analysis of the Flynn effect (1909–2013). Perspectives on Psychological Science, 10(3), 282–306.
- Bratsberg, B., & Rogeberg, O. (2018). Flynn effect and its reversal are both environmentally caused. Proceedings of the National Academy of Sciences, 115(26), 6674–6678.