The Flynn effect is the phenomenon by which performance on IQ tests in many countries has increased over the last 100 years or so.  A recent meta-analysis by scholars Jakob Pietschnig and Martin Voracek found full-scale test performance has been increasing by the equivalent of 0.28 IQ points a year, or 28 points over the last 100 years.

In my previous post, I argued that gains in brain size alone might explain about 5 of these points.

How do we explain the remaining 23 points?

It’s well known that schooling increases IQ scores (though probably not intelligence) by about 2 IQ points per year.  Given that people today get about four more years of schooling than they did 100 years ago, this explains another 8 points of the Flynn effect.

How do we explain the remaining 15 points?

A study by scholar Michel Duyme et al (hat-tip to commenter “Swank”) showed that kids adopted into  high socioeconomic homes score 13 IQ points higher than kids adopted into low socioeconomic homes.  Well, people who lived 100 years ago, with 8th grade educated parents in rural settings without electricity, indoor plumbing, television and internet access, were raised with far fewer socioeconomic advantages than people today, so we can expect that to shave off another 13 points (independent of the schooling effect).

Some might object that socioeconomic effects on IQ fade by adulthood, but that’s misleading.  Because socioeconomic effects are not, in my opinion, related to real intelligence, they do not strongly predict the adult socioeconomic environment which one must partly achieve using one’s true ability.  In other words, it’s not that socioeconomic environments stops affecting IQ, but rather that real intelligence starts affecting socioeconomic environment, causing socioeconomic environment to just become a redundant multiplier of real ability, creating the spurious impression that it has no effect.

However, when comparing people raised in different generations, the independent effect of socioeconomic factors don’t fade in adulthood, because they reflect differences in society that the individual has little control over.  What I am describing here is very similar to a model of the Flynn effect proposed by scholars James Flynn and William Dickens.

This leaves only 2 points of the Flynn effect unexplained.  I believe these remaining 2 points can be easily explained by what I call the Kaufman effect.  Years ago I noticed that new versions of IQ tests such as the Wechsler give a lot more coaching than older versions.  I began to wonder if this might cause people who take old IQ tests (after receiving extensive coaching on new ones) to score much higher than the old norm group on the old test (who did not have the benefit of this coaching) thus causing psychologists to overestimate how well new cohorts perform on tests compared to old cohorts.  I wasn’t the only one who thought of this, as apparently scholar Alan Kaufman wrote a paper on the topic.

So to sum up, I tentatively conclude that 5 points (per century) of the Flynn effect can be explained by rising brain size caused by nutrition, 8 points can be explained by more schooling, 13 points can be explained by socioeconomic advances, and 2 points are spurious (Kaufman effect):