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> IQ scores in most of the Western world began reversing sometime from the 90s to early 00s. [1] That link is for the US but similar stuff has also been observe

> IQ scores in most of the Western world began reversing sometime from the 90s to early 00s. [1] That link is for the US but similar stuff has also been observed in most of every Western/OECD nation. I wonder to what extent demographic shifts account for the observed reversal? It would be interesting to tease out what happens where composition remains constant, i.e. Poland. Though I doubt this explains the entire effect. > By contrast they're still rapidly increasing in the 'Global South'. seems like the Flynn effect doing its thing just as it did in western countries in the 20th century.

tptacek 9d ago
In what manner are you suggesting demography would be at issue here?
- adrianN 9d ago
  Intelligence is probably at least partially inheritable. If intelligent people have fewer children you expect intelligence to go down eventually.
  - tptacek 9d ago
    Just because smart people tend to have fewer children (because, for instance, they delay it due to schooling and career demands) doesn't mean genes for intelligence are necessarily being selected against.
    - gfrecvh 8d ago
      I dont understand what this phrase means. In the context of evolution, if something causes fewer copies of a gene, that's what selected against means.
      - tptacek 8d ago
        There's several reasons why this might not be the case, including that our premise is simple heritability and not genetic determinism, which is a different concept. I get where you're coming from now though.
  - kenjackson 9d ago
    Not that quickly though.
    - somenameforme 8d ago
      People beat around the bush but the heritability of IQ is extremely high, modern studies put it around 80%. So if one group has an IQ of 140 and another of 100, it'd imply about 32 of those 40 points are generally able to be explained by genetic factors. For some contrast that tends to be around, or a bit above, the estimated heritability for height. And so you'd see that easily in a single generation. It'd be akin to if tall people (or short people) began removing themselves from the gene pool in significant quantities. You'd immediately (well at least once people reached their adult height) see a significant increase or decrease in average heights.
      - kenjackson 8d ago
        But you’re seeing huge effects in three years, not a generation. The report notes “The change in mean scores that can be explained merely by the changing composition of the student body, however, never exceeds a few points.” So for Portugal for example about 10% of the drop might be attributable to immigration. But the other 90% to something else.
        somenameforme 7d ago
        The West is still mostly in the denial phase, so nothing is really being done, yet. As such, the effect is ongoing. The Flynn Effect (and its reversal) are measured in impact per decade. And numbers that seem small at first aren't necessarily so. For instance a 3 point decline per decade doesn't initially seem like much until you consider that IQ generally has an artificial mean of 100 with a standard deviation of 15. So 3 points per decade is 20% of a standard deviation per decade! That's enormous. In the past an IQ of 85 was considered mentally retarded. This was changed in 70s to make normal intelligence 'more inclusive.' That's another way of stating that 85 is still the best threshold for mental retardation. So a reverse Flynn Effect of 3 points/decade means you're talking about going from perfectly average to mentally retarded in 5 decades. And for many things, the effect will not be linear. If we take a test there's likely to be an exponential pattern where somebody of a certain IQ will get an answer correct e.g. 99% of the time, somebody a bit below 80%, somebody a bit below that 40%, and then somebody a bit below that will basically never get it correct beyond the point of statistical noise. Pair that with likely increased variance when looking at individual and relatively small societies, and it's fairly easy to imagine sharp swings being completely viable.

tptacek 9d ago

In what manner are you suggesting demography would be at issue here?

adrianN 9d ago
Intelligence is probably at least partially inheritable. If intelligent people have fewer children you expect intelligence to go down eventually.
- tptacek 9d ago
  Just because smart people tend to have fewer children (because, for instance, they delay it due to schooling and career demands) doesn't mean genes for intelligence are necessarily being selected against.
  - gfrecvh 8d ago
    I dont understand what this phrase means. In the context of evolution, if something causes fewer copies of a gene, that's what selected against means.
    - tptacek 8d ago
      There's several reasons why this might not be the case, including that our premise is simple heritability and not genetic determinism, which is a different concept. I get where you're coming from now though.
- kenjackson 9d ago
  Not that quickly though.
  - somenameforme 8d ago
    People beat around the bush but the heritability of IQ is extremely high, modern studies put it around 80%. So if one group has an IQ of 140 and another of 100, it'd imply about 32 of those 40 points are generally able to be explained by genetic factors. For some contrast that tends to be around, or a bit above, the estimated heritability for height. And so you'd see that easily in a single generation. It'd be akin to if tall people (or short people) began removing themselves from the gene pool in significant quantities. You'd immediately (well at least once people reached their adult height) see a significant increase or decrease in average heights.
    - kenjackson 8d ago
      But you’re seeing huge effects in three years, not a generation. The report notes “The change in mean scores that can be explained merely by the changing composition of the student body, however, never exceeds a few points.” So for Portugal for example about 10% of the drop might be attributable to immigration. But the other 90% to something else.
      - somenameforme 7d ago
        The West is still mostly in the denial phase, so nothing is really being done, yet. As such, the effect is ongoing. The Flynn Effect (and its reversal) are measured in impact per decade. And numbers that seem small at first aren't necessarily so. For instance a 3 point decline per decade doesn't initially seem like much until you consider that IQ generally has an artificial mean of 100 with a standard deviation of 15. So 3 points per decade is 20% of a standard deviation per decade! That's enormous. In the past an IQ of 85 was considered mentally retarded. This was changed in 70s to make normal intelligence 'more inclusive.' That's another way of stating that 85 is still the best threshold for mental retardation. So a reverse Flynn Effect of 3 points/decade means you're talking about going from perfectly average to mentally retarded in 5 decades. And for many things, the effect will not be linear. If we take a test there's likely to be an exponential pattern where somebody of a certain IQ will get an answer correct e.g. 99% of the time, somebody a bit below 80%, somebody a bit below that 40%, and then somebody a bit below that will basically never get it correct beyond the point of statistical noise. Pair that with likely increased variance when looking at individual and relatively small societies, and it's fairly easy to imagine sharp swings being completely viable.