I'm leaning against (but not convinced against) "intelligence genes" existing, at least to the point where some people have them and some don't, because evolution would likely have spread them to everyone.
But the science is much better for behaviors and temperaments being heritable. Some people are more comfortable sitting around only reading; some people are more likely to be comfortable fighting with abstract ideas; some people are more likely to be stubborn when they don't understand something and go at it until they figure it out. These things will affect what we measure as your IQ. My suspicion is that this is the source of the data for genetic differences in intelligence.
There's pretty robust evidence for intelligence (at least as measured by IQ) being heritable. There've been a bunch of identical-twin studies showing heritability factors as high as 0.8, which is roughly as heritable as height and among the highest of personality traits:
As for why these intelligence genes wouldn't spread throughout the population: it could be because they have other effects with a negative survival/reproductive value as well. Intelligence is often inversely correlated with confidence, for example, since people's expectations of themselves rise faster than their abilities. Confidence is heavily correlated with social skill and the ability to attract a mate, so highly intelligent people often face severe deficits in the mating game (I'm sure we all know someone who is brilliant but completely socially inept, and many of us have been that person).
Also, remember that the ancestral environment that humans evolved in significantly less favorable to intelligence than the modern one. 15,000 years ago, intelligence might be nice, but the ability to outrun a lion or take down a buffalo was far more important. It's only within the last 3-5 generations that intelligence has become essential to surviving in modern society (that's the whole point of Flynn's thesis), and evolution doesn't work so well on a timescale of 3-5 generations.
I'm leaning against (but not convinced against) "intelligence genes" existing, at least to the point where some people have them and some don't, because evolution would likely have spread them to everyone.
Why? What survivability/reproduction benefits does intelligence provide beyond a certain level, particularly in pre-modern society? If you were the daughter of serfs a thousand years ago, what good would a 150 IQ do you? Especially if there's some tradeoff involved, such as a correspondingly lower ability to socialize with your peers.
Especially if there's some tradeoff involved, such as a correspondingly lower ability to socialize with your peers
If there's any reason for an "intelligence gene" not being completely wide-spread, it would be this. Intelligence would have to have some very significant drawbacks. Some people have theorized that autism is when you get "overloaded" on those intelligence genes. It strikes me as unlikely this would really provide significant pushback against the intelligence genes completely dominating the gene pool, but I agree it's possible.
But why? Even if there's no tradeoff, what advantage would Albert Einstein have had over an average intelligence competitor for mates had he lived even a mere 200 years ago? Slim, I'd think. And who's to say he would have used that narrow advantage to win a mate of above average intelligence, vs. some other desirable characteristic?
Exceptional intelligence as a strong determinant of economic and social success (and thus presumably reproductive success) seems to be a relatively recent phenomenon in the human species.
Also, we need to take a larger view of what "intelligence" is. Take the longer view of human history, and we all are probably geniuses. Those intelligence genes have indeed spread across the gene pool. It's just new ones (or new combinations of existing genes) are now showing themselves and need time to prove their fitness.
The downvotes are unfortunate, because you're absolutely right. Intelligence has an associated cost. Natural selection will only spread "intelligence genes" if the benefit outweighs the cost. In pre-modern societies, this was likely not the case. "Social intelligence" was likely much more useful than abstract reasoning ability.
Even today, the Einsteins and Teslas don't do better in the mating game than the Brad Pitts and Kobe Bryants (likely much worse).
Edit: it would be interesting if HN handles were attached to downvotes. People would likely think twice before knee-jerk downvoting if they couldn't do it anonymously.
>because evolution would likely have spread them to everyone.
This is just your bias in what you think is more beneficial informing your belief that more intelligence = better survival. If this is the case, why aren't all apes just as intelligent as us? Has no ape simply ever had the luck to have a mutation that improved the carrier's intelligence? This is implausible. The answer is that one's environment determines what is selected for, and it is not the case that more intelligence is necessarily more fit for survival. Not even in modern times.
But the science is much better for behaviors and temperaments being heritable. Some people are more comfortable sitting around only reading; some people are more likely to be comfortable fighting with abstract ideas; some people are more likely to be stubborn when they don't understand something and go at it until they figure it out. These things will affect what we measure as your IQ. My suspicion is that this is the source of the data for genetic differences in intelligence.