Two weeks ago in the post The Big Brain Theory, I remarked on the inverse relationship between intelligence and fertility. Given the high heritability of intelligence (between 0.7 and 0.8 in most studies on a 0 to 1 scale), this might raise fears of breeding future generations of dullards. We can’t be sure how long this inverse relationship has held, but given that average human brain size (determined by skeletal evidence) peaked 20,000 years ago and since has shrunk by 150 cc, there is at least circumstantial evidence that it is a long-standing one. Yet, all through the 20th century (when IQ tests first became available), the numbers indicated something quite different: raw scores on IQ tests rose steadily decade by decade. This is the so-called Flynn Effect (see All in All I’d Rather Be Errol ).
There are clues in the data to possible reasons. Scores improved most at the low end, less in the middle to upper-middle ranges, and not at all on the high end. This suggests that, during the course of the 20 century, more and more test-takers reached their natural potential, perhaps due better living standards, better nutrition, and education targeted more at cognitive reasoning. High scorers of decades past already had reached their potential, so they didn’t improve. Since nutrition, living standards, and education no longer are getting better in the
First World, one might expect the Flynn effect to stall
at some point.
The point arrived with the 21st century. The Telegraph noted that in the
carried out in 1980 and again in 2008 show that the IQ score of an average
14-year-old dropped by more than two points over the period. Among those in the
upper half of the intelligence scale, a group that is typically dominated by
children from middle class families, performance was even worse, with an
average IQ score six points below what it was 28 years ago.” They are not
alone. Similar declines are turning up in a number of advanced nations. Thomas
Teasdale and David Owen in their 2006 paper Secular
declines in cognitive test scores: A reversal of the Flynn Effect analyze
the decline in scores in Denmark and Norway between 1998 and 2004: “Across all
tests, the decrease in the 5/6 year period corresponds to approximately 1.5 IQ
points, very close to the net gain between 1988 and 1998.” They surmise that
the Flynn effect is at an end in advanced countries, though still at work in
less developed ones. The reversal hasn’t shown up yet in the US, but, since the US
has lower absolute scores than Scandinavia
anyway (and some curiously Third World aspects to boot), Americans probably are
just a few steps behind.
Why It May Be Advantageous Not To Be Smarter Than One's Fellows