A fair idea: changes in memory changes as we age may explain the falling IQ trend Tim Graham/robertharding/REX/Shutterstock

We’re getting stupider – and our ageing population may be to blame. Since around 1975, average IQ scores seem to have been falling. Some have attributed this to the evolutionary effect of smarter women tending to have fewer children. But new evidence suggests population-wide intelligence could in fact be sinking because people now live longer, and certain types of intelligence falter with advanced age.

For about a century, average IQ scores in wealthy nations rose in a steady and predictable way – by about three points a decade. This is thought to be thanks to improvements in social conditions like public health, nutrition and education. Since this trend – called the Flynn effect – was first noticed in the 1940s, it has been seen in many countries, from the Netherlands to Japan.

But by 2004, researchers had begun to notice what appears to be a reversal in this trend, with average IQ scores going into decline. “The drop is around 7 to 10 IQ points per century,” says Michael Woodley of the Free University of Brussels in Belgium.


Working memory

The Flynn effect is supported by much evidence, and its reversal is still controversial. Even more controversial are some of the theories put forward to explain it.

Woodley and some others subscribe to the fertility hypothesis. This posits that the most educated people in Western countries have been having fewer children than the rest of the population, which is bringing intelligence down, generation by generation.

But it’s difficult to know if hypotheses like this are plausible because there is so little data. “A lot of this is piecing together patchy datasets from the past,” says Stuart Ritchie at the University of Edinburgh, UK. “We’re in the dark with historical trends [in IQ].”

Part of the problem is that the IQ tests have changed over time. Robin Morris of Kings College London and his colleagues have now found a way to get around this, by breaking down historical IQ tests into different component categories that are more easily compared.

Morris’s team looked through more than 1750 different types of IQ tests from 1972 onwards for two kinds of sub-tests: those that measure short-term memory, and ones that assess a person’s working memory.

Working memory is the ability to hold information for processing, reasoning and decision making. Unlike short-term memory, it’s a space in which you are able to manipulate the information, not just retain and repeat it.

When they looked at how people performed on these kinds of tests throughout time, the team saw a clear pattern. While short-term memory scores have risen in line with the Flynn effect, working memory ability has been declining, suggesting this particular kind of intelligence may be to blame for any reversal in IQ improvements.

Age effect?

Morris was reluctant to suggest why this might be the case, although he did say that remembering something in the short term is easier than keep things in working memory, which taps into a more complex battery of brainpower tools.

But his team did spot another trend among the historical IQ tests: an increase in the proportion of the people sitting tests who were aged 60 or older. Working memory is known to decline as we age, while short-term memory is usually preserved. In their study, Morris’s team write that the over-60s may be partly responsible for the decline in working memory scores in more economically developed nations.

“The idea that population ageing may be responsible is interesting and may serve as an alternative to the oft-proposed – but empirically little-supported – selective fertility patterns,” says Jakob Pietschnig of the University of Vienna in Austria. “It is a novel hypothesis that seems plausible and makes sense.”

But both he and Ritchie say they would like to see stronger and more specific tests of this idea, looking at elements of intelligence whose decline with age are well-established, such as processing time and reaction speed. Until then though, Ritchie warns that the concept of reversing IQs should be treated with scepticism. “This is speculative stuff and it’s only a handful of papers. Anyone drawing conclusions is jumping the gun.”

Journal reference: Intelligence, DOI: 10.1016/j.intell.2017.07.006

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