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There’s a popular idea, promoted by Malcolm Gladwell, that IQ only matters up to about IQ 120. In a previous post I showed that at least when it comes to income, IQ continues to substantially predict success well above 120. But perhaps the idea of an optimum IQ is correct, but Gladwell just set the bar too low. The smartest person I have ever corresponded with (a Promethean with an IQ around 180) insisted that the optimum IQ was about 130, and that not only were additional IQ points beyond 130 unhelpful to success, but that they actually might be harmful. He argued that up to half the people he knew with IQ’s above about 150 were failures by conventional standards working in unskilled jobs or not working at all and living off welfare. I found this very hard to believe, because from a Darwinian perspective, intelligence is the overall cognitive ability to adapt situations to your advantage (problem solve), so the idea that one could be so cognitively adaptable that one was maladaptive was paradoxical indeed. But that’s a philosophical argument; what does the actual data show?

For the first time we actually have data thanks to a longitudinal study of kids who before age 13 scored above the one in 10,000 level for their age group on either the verbal or math section of the old SAT. How this study knows what the one in 10,000 level is for 12-year-olds, I’m not sure, since so few 12-year-olds take the SAT, but I guess they assume they’ve tested virtually all the brightest ones in the country. Now, a score above the one in 10,000 levels implies an IQ of 156 or higher, so on average I’m guessing these kids were IQ 158 (one in 20,000 level) on either the verbal or math section. However since they averaged IQ 158 on only one section of the SAT, their IQs on the overall SAT would probably regress a bit to the mean, though not too much, since the overall SAT probably correlates above 0.9 with either subscale. Thus their overall IQs were probably 0.91(158 – 100) + 100 = 153.

So how successful were these kids by the time they were in their 30s? Table 1 in this paper reports that they created lots of art work, publications, and inventions, but the bottom line is that their primary annual incomes ranged from a pathetic $1,200 to a stratospheric $1,400,000, with a median of $80,000. Now if you read the small print beneath Table 1, it states that these dollar amounts were collected when they were 33, and have not been adjusted for inflation.

So let’s adjust for inflation. $80,000 circa 2002 is like $106,000 today. In other words, adjusted for inflation, the median of this highly gifted group was more successful than 95% of American 33-year-olds today.

According to scholar Arthur Jensen, the correlation between IQ and income is 0.4, so we should expect people with IQs of +3.53 Standard Deviations (SD) to have NORMALIZED incomes of 0.4(3.53 SD) = +1.41 SD. They actually exceeded this expectation because if we force income to fit the bell curve, then the 95 percentile is +1.73 SD.

So this idea that IQ has diminishing returns above 120 is simply wrong in a purely statistical sense. In fact I see no evidence of diminishing returns even above IQ 150. The only proviso I would add is that the kids were tested quite young (age 13) so it’s possible their adult IQs regressed to the mean, although the book The Bell Curve argued that IQ was essentially stable within measurement error beyond age 10; but we’re told from other research that IQ becomes more genetic post-puberty and in later maturity, so it’s quite possible that the IQs of these kids regressed well below 150 as genes loom larger in adulthood. However if the IQs of these kids regressed to the mean, then that makes the correlation with their incomes even stronger, since the two variables would more closely match in normalized standard deviation units, however we’re still left wondering how much money Americans with adult IQs above 150 are making.

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