Human intelligence has been increasing since the 1930s. This collective elevation of brain power is called the Flynn effect, named after James R. Flynn, an intelligence researcher who documented the observation.



The rise in worldwide IQ has been steady, but in recent years there have been signs that the trend is slowing down. The nature of the effect also depends on the specific aspect of intelligence being measured (as we’ve said, intelligence is more than one thing), and that’s where a new review has led to fascinating conclusions.

The review, just published in Intelligence, analyzed working memory scores from just about every relevant study since 1930. So which is it, is the world’s memory increasing or decreasing? The surprising answer is that it’s both.

Here* are the main results. Note that DS is Digit Span, CB is Corsi Block Span (what we’ve recreated digitally as Spatial Span), F is forward, and B is backward:

Digit Span is what it sounds like: viewing a series of numbers, then repeating it, either in the same order or in reverse order. Spatial Span is similar, except it’s a series of locations instead of numbers.

As you can see, the forward memory span tests (grey and pink lines) are increasing, consistent with the Flynn effect. However, the backward memory span tests (green and blue lines) are actually decreasing. In other words, people have gotten better at repeating a series in the same order they saw it, but have gotten slightly worse at the more complex task of reversing the order before repeating it.

The reasons for this trend are speculative and controversial. One possibility is that the same increases in overall health and nutrition that help boost simple task performance also allow people to live longer. This older population may maintain performance in simple tests as they age, but become impaired at complex tasks, such as the backward versions in this study. A more troubling possibility is that intelligent people are having fewer kids, which is gradually resulting in a gene pool that doesn’t code for brains with good memory performance.

We’ll surely see even more controversial and politically-motivated explanations as other scientists and the media struggle to explain these results. It’s worth keeping in mind that the effects are small, and as with all overall trends, they don’t say much about any individual.





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* Full citation (and image credit): Wongupparaj, P., Wongupparaj, R., Kumari, V., & Morris, R. G. (2017). The Flynn effect for verbal and visuospatial short-term and working memory: A cross-temporal meta-analysis. Intelligence, 64, 71-80.





This post was written by Mike Battista, a staff scientist at Cambridge Brain Sciences.