On IQ tests, people in Western countries (where the tests tend to be normed) are assigned a score above 100 if they are smarter than the average person of their generation, and assigned an IQ below 100 if they are dumber than their generation’s average. But how do psychologists decide what is average for each generation? By taking a representative sample of people in each age group and giving them a test. The problem is, not everyone who was part of your generation is alive to be tested, and the older you are, the more of your generation has died. And this attrition is not random. The correlation between IQ and longevity is said to be about 0.2, which means high IQ people have a small tendency to live longer (actually, since IQ tests are imperfect measures of intelligence, typically having a g loading of about 0.85 according to Arthur Jensen, the true correlation between general intelligence and life span would probably be 0.2/0.85 = 0.24).
Professor Bruce Charlton estimates that throughout most of history, less than a third of all human offspring survived. Charlton’s point is that historical IQ would have been genetically higher than modern IQ, partly because historically, those with IQ lowering mutations would have disproportionately died young at much higher rates than they do today.
Even without understanding the effects of mutations which accumulate over generations, a much simpler analysis is also worth doing. Let’s define an IQ of 100 not as the average IQ of every Westerner alive at a given time, but as the average IQ of everyone who was born at a given time. Today, both would be roughly the same, especially for young people, but back when only a third of children survived, there’d be a difference. How big a difference?
Let’s imagine there was a perfect correlation between IQ and life span. What would be the average IQ of the top third of the distribution? The top third is defined as an IQ above 105, however many people in the top third have IQ’s much higher than 105, so the average IQ of everyone in the top third would probably be about 122 (22 points above the average of every child born).
But of course, the correlation between general intelligence and life span is not perfect, it’s “only” 0.24, which we multiply by those 22 points to see that the average IQ of the living children would be 5 points above all of the children. Today, virtually all of the children do live, so genetic IQ would be about 5 points lower via the reduction of infant mortality alone.
Add to that the effects of dysgenic fertility, which according to Richard Lynn, has subtracted 5-8 genetic IQ points from Western populations since the mid-1850s and we’re pretty close to the 1 standard deviation decline that so many of us had doubted.
Of course this is a very oversimplified analysis so I don’t know how useful it is. And hopefully I didn’t make any math errors.