> One potential explanation was quasi-eugenic. As in the movie “Idiocracy,” it was suggested that average intelligence is being pulled down because lower-IQ families are having more children ("dysgenic fertility" is the technical term). Alternatively, widening immigration might be bringing less-intelligent newcomers to societies with otherwise higher IQs.

> However, a 2018 study of Norway has punctured these theories by showing that IQs are dropping not just across societies but within families. In other words, the issue is not that educated Norwegians are increasingly outnumbered by lower-IQ immigrants or the children of less-educated citizens. Even children born to high-IQ parents are slipping down the IQ ladder.

> Even children born to high-IQ parents are slipping down the IQ ladder.

It is expected that the children of high IQ parents have lower IQs than their parents (and that children of low IQ parents have higher IQs than their parents). That is just regression to the mean. How are they separating this from that effect?

> "That is just regression to the mean."

that would certainly be true if IQ were independently and identically distributed (iid), but the point here is that IQ is hereditable (not independent nor uniformly distributed) and so should show, in the long run, measurable durability in those differences across lineages. it's likely not reversion to the mean (or more to the point, the mechanism is not simply statistical in nature).

You're confusing concepts. Regression to the mean doesn't depend on any IID assumption. Conditional expectations are right in the definition! In the bivariate normal case (height, intelligence, etc), it's a necessary consequence of a correlation coefficient < 1.

>It is expected that the children of high IQ parents have lower IQs than their parents

Is it? Hadn't IQs been trending upwards for the past several decades?

Yes, but a lot of that was likely based on improving diets, especially during childhood.

Wouldn't parents with low IQ also have children who regress to the mean and have higher IQ?

They would. that is what happens with any heritable trait. It's clearly documented for example for height.

While genetics does play a role in intelligence, culture, context, as well as experiencing adversity that alters gene expression during development, and sometimes inherited over several generations...all these have enormous impacts on intelligence that do not imply that we are losing from the population the genes required to maintain high intelligence..

That suggests environmental chemical pollution or something cultural like cellphones addiction or too much Netflix or 4G radiation is dangerous. It will be interesting to see what it ends up being.

Not sure about that, aren't immigrants (on average) more educated than the native population?

Varies by country. The US has huge amounts of unselected immigration and makes no real effort to enforce its immigration laws so you get loads of Mexicans and before them Irish and Italians. All of those populations were negatively selected; the average immigrant to the US from those countries was less educated than the average for their country during the period of greatest immigration. Countries with selective or skills based immigration like Australia or Canada, with their points systems have immigrant populations that are a lot more educated than the native population or the average for the source countries.

I remember reading that a large percentage of US companies are founded by immigrants. About 40-50%? Can’t recall sources , but it was a much larger number than what I thought.

then there is a difference between general immigration and war refugees

There is a documentary about that: https://en.wikipedia.org/wiki/Idiocracy