Jelte Wicherts and I both had American data that showed little-to-no gains in the US, although his research was much more comprehensive as he used the WJ norming samples. I am in the midst of resubmitting my article/data, and I imagine Jelte is as well.
Although the apparent demise of the Flynn effect is bad for the world, it is a relief to me, as every attempt at logical explanation for it fails. In reality humans should be getting dumber; the birth rate is higher and the generation size is smaller at the low end of the curve. If there is a hereditable component to intelligence – even a non-genetic one – the differential birth rates and generation sizes should result in a lowering of the mean.
I thought it was well established that the Flynn Effect worked on the left side of the distribution only. Is this being ignored or did I miss it?
There is a well known correllation between height and IQ but it is also well known that this phenomenon is a product of confounding two populations: the “normal” and the defective. Those who have no genetic diseases or have not had a severe envioronment that stunted them, have no correllation between height and IQ. Those with bad genes/aneuploidy or a midieval peasant upbringing will likely be short and stupid.
I have long assumed that the elimination of these factors was what was powering the Flynn Effect. Obviously this sort of reform can’t go on endlessly. The irony is that the as these problems are cleared up the phenotype comes to ever more closely express the genotype. This is sometimes called the Hernstein Effect I believe.
it is also well known that this phenomenon is a product of confounding two populations
I’m sure including the residual population with pathologies would strengthen the correlation, but I believe the effect remains even when they are eliminated. See here for example (top hit if you google for ‘correlation between height and IQ’).
If you buy the heritability of intelligence at all, you have to guess that the Flynn effect represents the sum of at least two different forces: Dysgenic forces pushing the average scores down by X, and some other set of forces pushing the average scores up by Y, where Y>X. The end of the Flynn effect might just be those dysgenic forces overpowering the other forces–the pool at the bottom has expanded so that X>Y, say.