Here's a strange fact. If you took a typical adult from the 1950s and scored their IQ test against today's norms, they would likely land well below 100. On paper, a whole generation would look below average next to their grandchildren.
That doesn't mean your grandparents were less capable than you. It means something odd and fascinating happened to IQ scores across the 20th century. It's called the Flynn effect, and once you understand it, you'll never read an IQ score quite the same way again.
What is the Flynn effect?
The Flynn effect is the long-term rise in average IQ test performance that was observed in many countries throughout much of the 20th century. It's named after James Flynn, a New Zealand-based political scientist and researcher who documented the pattern in detail during the 1980s, although other researchers had noticed hints of it earlier.
The headline finding is simple: on many tests, average performance rose by roughly three IQ points per decade. Over several decades, that adds up to a large shift.
The reason most people never notice is that IQ tests are periodically re-normed. The average score is always reset to 100 against a fresh sample, which quietly hides the rising raw performance underneath.
Why your grandparents would score "below average"
To see how this works, remember that an IQ score isn't a raw grade. It's a comparison with a reference group. If you're new to that idea, our guide on how IQ tests work explains norming in more detail.
Now imagine scoring two people against the same modern norm group:
| Test taker | Scored against their own era's norms | Scored against today's norms (roughly) |
|---|---|---|
| Average adult, 1950s | 100 | Noticeably below 100, often estimated in the 70s to 80s |
| Average adult, 1980s | 100 | Somewhat below 100 |
| Average adult today | 100 | 100 |
The exact figures depend on the test, the country and the decades compared, so treat the middle column as an illustration rather than a precise conversion. The point is the direction: older generations solved fewer of the same puzzles, especially abstract ones.
Flynn himself found this conclusion implausible taken literally. A population whose average sat in the 70s by modern standards would struggle with everyday tasks, and that plainly wasn't true of people in the 1950s. So the gains must reflect something more specific than "everyone got smarter".
Where the gains showed up most
One of the most revealing features of the Flynn effect is that it wasn't even across all types of test.
- Largest gains: abstract reasoning tasks, such as Raven's Progressive Matrices, and questions about similarities ("How are a dog and a rabbit alike?").
- Smaller gains: vocabulary, general knowledge and arithmetic.
This pattern matters. The biggest rises came on tests of fluid reasoning, the ability to solve unfamiliar problems on the spot. Gains in crystallised knowledge, the facts and words you've accumulated, were more modest. Our explainer on fluid vs crystallised intelligence unpacks that distinction.
What caused the Flynn effect?
There's no single agreed answer, and researchers still argue about the balance. But several explanations have strong support, and most experts think they worked together.
Better nutrition and health
Improvements in diet, prenatal care and childhood health supported brain development. Average height rose over the same period, and many researchers see the two trends as linked to similar causes.
More, and different, schooling
People stayed in education longer, and schooling increasingly emphasised abstract thinking, classification and hypotheticals. Those are exactly the skills that many IQ subtests reward.
"Scientific spectacles"
Flynn's own favourite explanation was cultural. He argued that modern life trained people to wear what he called "scientific spectacles": to sort things into abstract categories and take hypothetical questions seriously. A farmer in 1900 might answer "What do a dog and a rabbit have in common?" with "You use a dog to hunt rabbits." Practical, and smart, but not the abstract answer the test wanted.
Smaller families and richer environments
Smaller family sizes, more access to books and media, and more cognitively demanding work may all have contributed. Flynn and economist William Dickens also proposed a model in which small environmental advantages get multiplied over time, as people seek out environments that further stretch their abilities.
What the Flynn effect does not mean
It's easy to draw the wrong lessons, so here are a few clarifications.
- It doesn't mean past generations were less intelligent in daily life. They were solving different problems, often with skills IQ tests don't capture well.
- It doesn't mean the gains were genetic. The changes happened far too fast for genetic shifts to explain them, which is strong evidence that environment can move test scores.
- It doesn't mean IQ tests are useless. Within a generation, IQ scores still predict outcomes such as academic performance reasonably well. The Flynn effect shows they measure skills shaped by the world people grow up in.
- It doesn't mean the rise continues forever. In several countries the gains have slowed, stopped or even reversed in recent decades.
That last point is a story in its own right, which we cover in the reverse Flynn effect explained.
Why it matters for your own score
The Flynn effect has a very practical consequence: the age of a test's norms affects your result.
If a test was normed many years ago, the population it compares you with performed less well on average than people today. That can inflate your score. This is one reason good test publishers update their norms, and why an IQ score always needs to be read in the context of the test that produced it.
It also offers a hopeful message. Average performance on reasoning tests shifted by a large amount within a few generations, driven largely by education, health and the habits of thought people practise. Thinking skills are shaped by experience more than many people assume.
Curious how your reasoning compares with people today? You can take our free IQ test, which is scored against modern norms, and see roughly where you land.
The bottom line
The Flynn effect describes the steady rise in IQ test performance across much of the 20th century, roughly three points per decade on many tests. Your grandparents would score below average against today's norms, but not because they were less capable. The gains reflect better health, more schooling and a world that trains abstract thinking, and they show how strongly environment shapes what IQ tests measure.
FAQ
Who discovered the Flynn effect?
The effect is named after James Flynn, who documented rising IQ scores across many countries in the 1980s. Earlier researchers had noticed signs of it, but Flynn's work brought it to wide attention.
How much did IQ scores rise because of the Flynn effect?
On many tests the average rose by about three points per decade during the 20th century. Gains were largest on abstract reasoning tasks and smaller on vocabulary and general knowledge.
Is the Flynn effect still happening?
In some places it appears to be continuing, while in others, including several Scandinavian countries, gains have stalled or reversed since around the 1990s. Researchers are still studying why.
