Reaction times–a reliable marker of general intelligence–have declined steadily since the Victorian era from about 183 milliseconds to 250ms in men, and from 187ms to 277ms in women.
The slowing of our reflexes points to a decrease in general intelligence equivalent to 1.23 IQ points per decade since the 1880s or about 14 IQ points overall, researchers said.
Actual IQ scores from different decades cannot be directly compared because people today enjoy better teaching, health and nutrition which would help improve their results, the scientists explained.
But the reaction times signify that the genetic component of general intelligence–which leads to the type of creativity and invention typical of the Victorian era–has been dwindling over the past century.
Dr Michael Woodley, who led the study published in the Intelligence journal this month, identified the trend by comparing reaction times from trials conducted by Victorian scientists against those carried out in recent decades.
Our declining intelligence is most likely down to a “reverse” in the process of natural selection, he explained. The most intelligent people now have fewer children on average than in previous decades, while there are higher survival rates among people with less favourable genes.
“The pressures of modern life, a nine-to-five modern lifestyle, have created all these pressures against very smart people having break-even numbers of children,” he said.