Just think of a kid adopted out of the nursery by two smart parents. Is that kid: A) more likely to score higher on tests, B) more likely to score lower on tests, or C) exactly as likely to score high/low on tests no matter who his adoptive parents are?

Approximately C. See IQ adoption studies like https://www.gwern.net/docs/iq/2021-willoughby.pdf#page=6 for the scatterplot for adoptive parental IQ vs adoptee.

I am imagining if any couple could adopt; taking the same kid (which of course we cannot do) and having them raised by a couple of college professors vs. a couple of meth addicts living in a drug den - I would be absolutely stunned if there were no difference in what kind of test scores the kid would get growing up.

There were a bunch of studies on identical twins who were adopted in different families, so it can be tested. I think they showed the same thing (kids parents more important than adoptive parents) but I'm not certain and can't quickly find the studies to verify.

Here, unfortunately that doesn't get around the confounding factor that only parents deemed suitable candidates by adoption agencies are able to adopt and raise children, so they are not an accurate cross section of all possible parents. I don't dispute that there may be a threshhold at which some innate traits take over (or fail to do so) from the benefits of a nurturing environment (which may have diminishing returns past a certain point).

Not only do we have evidence that the heritability of intelligence is high, from twin studies, but also: adopted siblings' intelligence is only slightly correlated-- barely better than strangers. Biological sibling intelligence raised in the same house are very closely correlated.

This seems to imply once parenting is "good enough", these types of environmental effects don't affect IQ in adulthood to a significant degree.

Perhaps the most interesting finding: parenting does seem to affect IQ scores in early childhood more than genetics, but by adulthood the strengths of these relationships have completely reversed.

Into adulthood I find it very difficult to say whose intelligence is "better" in anything close to an objective way. While the results of genetic influence may become evident in some kinds of tests, it is just as easy to conceive tests at which the same candidate would fail yet someone else with a lower IQ might do very well. The possibilities are endless and I really don't think outside of cases of abuse or neglect creating specific developmental deficiencies as you mentioned that we can meaningfully quantify intelligence generally. If we could better do so I think the influence of parenting might still become more evident than what we see in IQ tests.

> No one is arguing that.

Well, my initial comment was simply an attempt to make the point that parenting can have an effect whether or not genetics do and the reply I got was "look it up".

Intelligence is difficult to quantify and define. BUT: peoples abilities on a wide variety of tasks are positively correlated. This strongly implies there is some kind of underlying factor that makes you better at "all things", and this is what IQ tests attempt to measure.

e.g. see: https://en.wikipedia.org/wiki/G_factor_(psychometrics)#/medi...

Of course, the correlation coefficients are not 1; so of course someone can have a lower IQ than you but far outperform you in one of these tasks.

The results from IQ tests seem to be very strong predictors of many outcome measures later in life-- indeed, more than parenting seems to predict these things.

edit: from the wikipedia article on g factor:

" Research indicates that tests of g are the best single predictors of job performance, with an average validity coefficient of .55 across several meta-analyses of studies based on supervisor ratings and job samples. The average meta-analytic validity coefficient for performance in job training is .63.[76] The validity of g in the highest complexity jobs (professional, scientific, and upper management jobs) has been found to be greater than in the lowest complexity jobs, but g has predictive validity even for the simplest jobs. Research also shows that specific aptitude tests tailored for each job provide little or no increase in predictive validity over tests of general intelligence."

The usual analogy is that you can always lower the IQ of a child by hitting them on the head with a hammer or by lacing their food with lead powder. But no intervention has been found that raises IQ.

Since almost no one, without deliberate intervention, experiences an environment completely free of known factors that are detrimental to IQ compared to the optimum environment, it's purely a semantic game to say that there are no known interventions that increase IQ.

The South Korean twin had an IQ of 100 (Slightly below average for her country) and the US twin had an IQ of 84. (Significantly below average)

So, still no evidence of an intervention that can increase IQ, but plenty of evidence that it can be lowered.

Edit: This is my opinion on what I consider a fact based on a fair bit of research and first hand observation. People are free to disagree with this but if they've made it to this point still holding that opinion then I think it's unlikely anything I could say would change their minds.

You replied to my comment of "genes don't even have to boost intelligence for kids from smart parents to be more likely to be intelligent" with "Unfortunately intelligence is mostly genetic, look it up..." I'm not sure why you used the word "unfortunately" or what you want me to look up since you didn't even appear to be responding to what I wrote.

> but if they've made it to this point still holding that opinion[...]

I'm confused as to what this is intended to mean.

> [...]then I think it's unlikely anything I could say would change their minds.

How can you know to what extent anyone reading this has made up their mind about anything?

Given the difficulty of defining an objective measure for intelligence and the confounding factors of upbringing, I think it is almost impossible to find a good source on this sort of thing. Why should we go on a snipe hunt to prove your point?

IQ is not a perfect metric, but it is a decent predictor of many outcomes.

> the confounding factors of upbringing

This is why you do twin studies and studies using adoption as a control. We have quite a wealth of them. A good critical review of the evidence on both sides is here: https://humanvarietiesdotorg.files.wordpress.com/2013/05/the...

* Identical twins have a much higher correlation coefficient of intelligence than fraternal twins (table 5).

* Identical twins raised apart have a correlation coefficient for intelligence of around .73 (MZA studies).

* Correlation coefficients of IQs of biologically unrelated siblings raised in the same adoptive environment are approximately 0.

There are obvious confounds the paper mentions:

* Fraternal twins may not have identical environments.

* Adoptive environments may not represent typical parenting environments.

* Some of these studies have had poor controls and practices.

* Distribution of results from studies looks bimodal which indicates something weird is going on.

Still, even if you take the most critical view I think you have to accept that there is still substantial heritability of intelligence. These results cannot be convincingly explained any other way.

To ellaborate: aren't there unintended consequences of very high intelligence?