What is intelligence? This may sound like a straightforward question with a straightforward answer — the Oxford English Dictionary defines it as “a capacity to understand” — but that definition itself raises an increasingly relevant question in the modern world. What happens if the extent to which we can practically apply that capacity is diminishing? Evidence is mounting that something exactly like this has been happening to the human intellect over the past decade or so.

Nobody would argue that the fundamental biology of the human brain has changed in that far-too-short time span. However, across a range of tests, the average person’s ability to reason and solve novel problems appears to have peaked in the early 2010s and has been declining ever since.

When the latest round of analysis from PISA, the OECD’s international benchmarking test for performance by 15-year-olds in reading, mathematics and science tests, was released, the focus understandably fell on the role of the Covid pandemic in disrupting education. But this masked a longer-term and broader deterioration.