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My recent post about how readers of brainsize.wordpress.com have an average IQ of 147 was damaging to the self-esteem of many readers. For example a reader named “Andrew” wrote:

I suppose I’m the dunce around here with an IQ of ~120!

Intelligence is relative, so even though an IQ of 120 is higher than 90% of the U.S. population, when you find yourself on a blog like this one where the average IQ is said to be 147, an IQ of 120 can feel extremely low.

Well the good news for all the “Andrews” out there is that my readers do not have an average IQ of 147 after all. That figure was arrived at using remarkabley indirect evidence. A poll I conducted found that the average reader was about 0.62 standard deviations taller than others of their demographic group, and since height is thought to correlate 0.2 with IQ, I simply dividing their average height (+0.62 SD) by 0.2, to estimate their average IQ to be 3.1 SD above the mean (IQ 147).

There were a couple problems with this however. For starters, as more readers have started to vote in the height poll, it seems the average reader is 0.43 SD taller than normal, not 0.62 SD as previously estimated. Secondly, while 0.2 is the correlation between IQ and height, what I really want is the correlation between height and g (general intelligence) and I want it corrected for reliability. This figure would be about 0.26.

So now with the revised height (+0.43 SD) divided by the true g loading of height (0.26), it seems Brain Size readers have an average general intelligence of 1.65 SD above normal. In other words, Brain Size readers average an IQ of 125. This is a far more believable figure than 147, which is about what you’d expect from the average academic Nobel Prize winner.

Many people are confused by why I divided the height of my readers (+0.43 SD) by the true g loading of height (0.26), instead of just multiplying it. For example, NBA players are ridiculously tall (perhaps +3.88 SD taller than other American men in their age group). If I wanted to estimated the average IQ of NBA players from their average height, would I divide their height by the g loading of height? If so, their estimated intelligence would be +14.92 SD (an average deviation IQ of 324 making the average NBA player more than 100 IQ points smarter than any person who ever lived!) So instead of dividing their height (+3.88 SD) by 0.26, I would multiply, which gives an estimated intelligence of +1 SD (IQ 115).