Intelligence is probably at least partially inheritable. If intelligent people have fewer children you expect intelligence to go down eventually.
In the context of evolution, if something causes fewer copies of a gene, that's what selected against means.
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.
And that's before you get to the slippery definition of what "heritability" means. It does allow you to use the words "explained by genetic factors", but in a very specific way, one in which the number of hands you have isn't similarly explainable, but how much TV you watch is.
And we've already had this conversation and in spite of your obsession with bias confirming 'blogs written by very smart people' - the heritability of TV watching is zero. Actually lol looking this up, it turns out there has indeed been a study done on this [1], as part of a study on the heritability of sedentary behaviors in general. And yeah, it's zero.
You are right that the number of hands you have is not heritable, but is only because heritability is defined as the percent of variation in a trait, between populations, that can be explained by genetic factors. When basically everybody has two hands, there is no variation, so there is no heritability to measure. It'd be like discussing the heritability of hair color if everybody had black hair. It's not some sort of clever insight or retort, though I'm certain your blogs framed it as the wittiest thing ever.
The one specific claim you make here, that height has heritabilities measured at around 10%, is false. In fact, the comparisons between the steadfastly high heritabilities of simple physical phenotypical traits like height and complex behavioral characteristics is a big part of the current scientific conversation about behavioral genetics.
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.
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.
But if you go into the past and just look at well educated, well nourished individuals, regularly exercising their brain - then their cognitive ability was likely comparable, if not higher, than that of a typical modern era individual. But that sample made up a very small chunk of all people whereas today I'm comparing them to the norm.