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Psychologists have long noted that intelligence, along with its biological correlates like brain size and processing speed, increases with age from infancy to childhood to adolescence, before plateauing in incipient adulthood (about age 16).    Thus historically, if a young adult had the reading skills of a 9-year-old, he was said to have an IQ of 56, because his mental age was only 56% as advanced as his chronological age (16+).  In other words, adults with IQ 56 were thought to be as smart as 9-year-olds.

However an intelligence expert astutely caught the error in this logic.  A 9-year-old only had 9 years to acquire those reading skills, while the IQ 56 adult had 16+ years. So an average 9 year old is actually smarter than an IQ 56 adult.  Instead the expert suggested that more direct measures of intelligence, like reaction time, would be a better basis of comparison than acquired skills.

In a previous post I noted that modern adults (age 16 to 25), in Western countries, have simple reaction times of about 273 milliseconds and that the standard deviation for simple reaction time is thought to be 160.4 milliseconds.  Now I’ve read that in this study (which I can’t access), it’s reported that the average British 9-year-old has a simple reaction time of 371 milliseconds.  Now assuming that’s true, and assuming the sample is representative and, reaction time measured the same way, then the average nine-year-old is 0.61 standard deviations slower than an adult.

However because reaction time is a very crude measure of general intelligence, it can greatly underestimate the intelligence difference between groups of people, so despite some earlier objections on my part, experts would probably consider it reasonable to divided this 0.61 SD difference by 0.54 which increases it to 1.13 SD..  In other words, using simple reaction time to estimate intelligence, 9-year-olds are as smart as an adult with an IQ of 83.  

So just as reaction time studies estimate modern people are about 1 SD dumber than the Victorians, by the same measure, nine years olds are about 1 SD dumber than adults.  In other words, according to reaction time, the average adult today is about as dumb as a Victorian 9-year-old.

How can this be?  Well, perhaps reaction time studies overestimate the intelligence of 9-year-olds and Victorians because both groups are much shorter than modern adults and thus the nerve impulses have less distance to travel.  Or maybe 9-year-olds are much less developed in other properties of the brain, so reaction time overestimates their intelligence.

But even if dysgenics and mutation load has driven our general intelligence to become as low as a Victorian 9-year-old’s, better nutrition has greatly improved brain size by as much as 1.63 SD (and perhaps other properties of the brain too) and this may be creating huge gains in non-verbal intelligence. Ironically, genetic brain size has probably declined by over 0.4 SD.

Remember, general intelligence is only one part of intelligence.  The nutrition driven improvements in non-verbal IQ may be greatly enhancing our technological skills, but the rest of our intelligence has declined, as we see in our dumbed down media, devoid of critical thinking skills, and the horrific vitriolic comments littering most internet news stories.

It may be hard to believe that we’ve become so genetically dumb, but as the brilliant philosopher Christopher Langan once said, the stupid don’t know that they’re stupid.  And therein lies their stupidity.  It also explains why so few people read this blog.  I’m just too intelligent for the modern mind. 🙂