On a recent post about genes and IQ, commenter JL objected to my statement that the brain is a uniquely plastic organ, and that humanity’s astoundingly extended childhood appears to exist to maximize that plasticity. More specifically, JL objected to my conclusion that given the brains plasticity, we would expect intelligence (and all the other factors included in IQ testing) be incredibly responsive to the environment.
While IQ is highly malleable in principle, in practise it is one of the more stable human characteristics across the lifetime.
The nice thing about disagreements like this is that we can look at the data (what data there is).
I suggested that JL was overestimating the degree of individual stability represented by a high correlation between earlier and later tests. Amusingly, both JL and Bryan Pesta (who will likely show up on the comments here to attempt to convince the world I have no idea what I’m talking about by saying I have no idea what I’m talking about without actually demonstrating it–dude needs a new hobby) referred me to the data set I was looking at when I made the comment. Only I was looking at the paper that included a visualization of the data (pdf).
The following is a scatterplot of IQ scores of Scottish people tested as schoolchildren in 1932 and again much later in life.
The center teal line is where scores would fall if they were the same on both tests. The lines to either of side of those represent a difference of 15 IQ points, which is the standard deviation for an IQ test.
As you can see, there is significant deviation from the age-11 scores. There are people whose scores would have registered as mentally retarded at age 11 who registered as average intelligence at age 80. Former near-geniuses are now average. Even excluding the outlier, those of average IQ score in 1932 (red line) span three standard deviations of IQ scores decades later.
It’s also worth noting that starting this sort of comparison at age 11 cuts off much of the more extreme variability that is seen in early childhood. And that, my friends, is what “one of the more stable human characteristics across the lifetime” looks like across much of the lifetime.