IQ Across a Lifetime: What Holds Steady, What Slips and What Can Still Improve

In 1932, Scotland gave an intelligence test to nearly every 11-year-old in school that day, essentially all children born in 1921, some 87,000 in total. Decades later, psychologist Ian Deary's team retested a group of those children, by then around 77. The correlation between the score at 11 and the score at 77 was roughly 0.63. For a gap of 66 years that is high, though far from perfect. It sums up the whole topic: IQ is fairly stable, but it is not frozen.

A relative number, not an absolute one

First, it helps to separate two ideas. IQ is a relative measure: it compares you with people your own age. If a 70-year-old has an IQ of 110, that means she performs better than most 70-year-olds, not that she performs the way she did at 25. So your standing in the group can stay nearly the same even while your absolute performance on certain tasks shifts over the years.

Still, stability grows with age. The correlation between two measurements a few years apart is higher in adulthood than in childhood, when the brain develops fast and scores bounce around more.

Fluid versus crystallized

Psychologists Raymond Cattell and John Horn distinguished two broad families of ability, and each ages in its own way:

• Fluid intelligence: reasoning through new problems, spotting patterns, juggling information in your head. It tends to peak in young adulthood and decline gradually afterward.
• Crystallized intelligence: vocabulary, general knowledge, accumulated experience. It usually holds steady, or even improves, well into later life.

Processing speed, meaning how quickly you handle simple mental tasks, is among the things that falls most with age, according to researchers such as Timothy Salthouse. That is why a timed test penalizes an older adult more than one without a clock. Think of your uncle, who takes longer than your 20-year-old cousin on a shape puzzle but sorts out a real-life problem in a minute because he has seen it before. That is not "thinking worse." It is making up, with knowledge and judgment, for something a pattern test does not measure.

Adolescence: a brain still under construction

There is more movement in the teen years than people used to think. A 2011 study in Nature by Sue Ramsden, Cathy Price and colleagues retested teenagers about four years later and found changes of several points, in some cases up to 20, in verbal and nonverbal IQ, alongside changes in brain structure. There was debate over how much of that variation is real and how much is measurement noise. Even so, it shows the brain is still being built at that age.

Can schooling raise IQ?

There is reasonable evidence that it can, a little. A 2018 meta-analysis by Stuart Ritchie and Elliot Tucker-Drob estimated that each additional year of education is associated with a gain of 1 to 5 IQ points. Researchers still argue about how much of the effect lasts and how much is causal, but the direction is clear: keeping your brain busy with demanding material matters.

Something similar shows up in later life. People who keep reading, learning, taking on demanding work and staying socially engaged tend to show slower decline. Researchers still debate how much of that is cause and how much is simply that healthier, more curious people do these things anyway. The habits are cheap and worth having either way.

What to expect if you retake a test over the years

If you take the same kind of test at different points in your life, it is normal for the results not to match exactly. Part of the difference is real and part is measurement error: even professional tests carry a margin, so a 105 today and a 110 tomorrow do not mean your brain changed. Sleep, stress, motivation and having practiced similar exercises before all play a role.

That is why psychologists speak in confidence intervals rather than exact figures. Thinking of your score as an approximate zone, not a number carved in stone, is healthier and truer to the science.

If you are curious how you are doing today, take our test and keep the result. Repeating it in a few years is a fun way to see what moved.