A lot of people in this thread have indicated that it was normal within their families to be gifted. This goes against my limited understanding of genetics and intelligence, but it may make sense given that this forum acts as a selection bias.
In general, my understanding is that the offspring of people who are exceptional within their populations will regress toward the mean of their population norms. So, for instance, if a bunch of people with IQs 2+ standard deviations above their population average were to mate, on average their offspring wouldn't have IQs as high as their parents. But, some of the offspring would have IQs just as high or higher than their parents (due to variance) and those are the parents represented on this forum.
If this is correct, then I think it may have an important impact on what expectations these parents may have regarding their grand children.
If this is wrong, someone please tell me why.
It's wrong, or at least, it's misleading because it has hidden assumptions that are incorrect. No time right now to write a long post, and writing a short one may be even harder, but let me try...
I think the easiest way to see that there must be something wrong with the general argument
In general, my understanding is that the offspring of people who are exceptional within their populations will regress toward the mean of their population norms. is to ask yourself "which population?". Suppose you took a two humans who, considered as humans, were perfectly average - let's take in height, since we're dealing with a general argument not something specific to giftedness. If you consider them as members of the population of humans, you expect their children to be, on average, the same height as them, since they're already at the population mean. However, you can just as easily consider them as members of the population of primates, or mammals, or if they happen to be Danes (so they're below the mean height for that population!) as Danes. Depending on which population you consider, the "population mean" to which you expect their children's height to regress can be quite different. However, the children are the same people with the same inheritance, so this is obviously nonsense. Hence we should be suspicious about the original general statement.
What is true is that if you:
- took a random sample of adult humans
- measure their IQs
- caused them to mate :-)
- measured the IQs of the offspring
- then looked at the adults who'd scored very highly, and checked the IQs of their offspring;
Then you'd undoubtedly find that the offspring of highscoring parents tended to have lower IQs than their parents. A major cause of this (note hedging wording :-) is measurement uncertainty and error: since the highscoring parents were, prior to scoring highly, expected to be average, our Bayesian expectation of their "true" IQ, or if you like what we'd expect them to score if we tested them again, should not be the same as the previous measurement but somewhere in between that and the mean. Therefore it shouldn't be surprising to find their children in that range - even if IQ were 100% heritable with 100% certainty, e.g. a child's IQ was always identical with its mother's, we'd expect that.
However, the situation is not very relevant to what we see here. Most obviously, parents who self-report as gifted here do so on the basis of lifelong experience of being them, not on the basis of a single IQ test. Measurement uncertainty is therefore not very relevant. It's possible that they have a non-standard definition of gifted in mind, but then, they're likely to apply the same non-standard definition to their children, so this hardly matters.
We *also* have a great deal of selection bias, as you say. Moreover, presence here suggests an unusual degree of interest in optimising the environmental factors affecting children's intellectual development, whatever they are, which
might also be expected to play a role in tending to cause children of parents here to have higher IQs than children of parents who were similar except that they were not here.
There may, of course, be genuinely interesting genetic things going on - IQ, in as far as it is genetic at all, is clearly influenced by many many genes, and what you expect of children of people with a given IQ will depend on just which relevant gene variants their parents had, and then what really happens in a population will depend on the extent to which people who actually produce offspring tend to have the same gene variants as one another... no doubt it gets very complicated, and we aren't close (and probably never will be) to really understanding the genetics of intelligence.
Here is a post I wrote about similar issues a few months back.
