In November 1984, the social scientist James Flynn received a package of IQ test results in the post, comparing two generations of Dutch eighteen-year-olds. The younger group scored far higher than the older one, and when Flynn checked further countries the same pattern kept turning up.
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By the time he had gathered data from almost thirty countries, Flynn found IQ scores climbing everywhere he looked, by roughly 0.3 points a year, or about three points a decade, for as far back as the tests went. Psychologists now call the pattern the Flynn effect, and treat it as settled fact rather than theory.
Why the pace ruled out genes
Evolution does not move fast enough to lift a population’s average intelligence within a few generations. A shift this large, this quickly, pointed to environment rather than heredity, which is what made the finding hard to square with earlier research on twins.
Kinship studies of identical twins raised apart had shown those twins growing up with strikingly similar IQs, evidence that seemed to support a heavy genetic influence on intelligence. The psychologist Arthur Jensen built a model from that data in 1973 that put roughly 75% of the variation in individual IQ down to genetic differences. Applied to a large national gain across a single generation, that same model implied an environmental change so extreme it did not seem possible.
The economist William Dickens proposed a resolution: a small genetic edge in a trait can pull a person into a better environment for developing that trait, and the two effects then reinforce each other. A child with even a slight edge at schoolwork gets more encouragement, studies more, and lands in a stronger academic track, which then does more work than the original genetic edge ever did on its own. Kinship studies pick that up as a genetic effect because the enhanced environment tracks the genes, even though genes only started the process.
What Flynn linked the rise to
Flynn linked the rising scores to modernisation itself: more years spent in education, and more jobs demanding analytic and abstract thinking, both of which the tests reward. The New Yorker set out how the numbers translate across generations. An American born in the 1930s with an IQ of 100 would, run forward through the effect, have grandchildren scoring close to 120.
Run backward, that same logic puts the average American schoolchild of 1900 at around 70, a score that would today fall below the threshold usually associated with intellectual disability. Flynn saw only two readings. Either today’s children were far brighter than their parents, or IQ tests were not always good measures of intelligence.
The debate the finding reopened
Flynn, who taught political studies at the University of Otago in New Zealand, found his data complicating a separate argument about race and IQ that had run for years before his 1984 discovery. Writers who treated group differences in IQ scores as fixed and largely genetic had to reckon with a trait that could shift by more than a standard deviation in two generations.
Flynn did not treat the story as finished. In a later book, Does Your Family Make You Smarter?, he found that a family’s effect on a child’s IQ fades by around age 17 for most of the test, though it lingers into the mid-20s for vocabulary. Genetic influence grows stronger as a person ages, he told the Guardian, but that growth adds to what environment does rather than replacing it.
