You just don’t. Nobody does.
People like to speculate on the IQ of various celebrities. A popular one recently was Sarah Palin (“likely somewhere between 110 and 115“). We may also have “learned” that Beyonce’s IQ is 124 (or is it 110?) And nobody can agree on Einstein’s putative IQ, except that it was somewhere between “only” 160 and 250 or more.
Online, I’ve seen the same group of nerds who enjoy self-diagnosing Asperger’s report their IQ scores as 180 or more (200 is a popular number). There are two possible reasons a person might say this:
- They took some fake test on the web.
- They are making it up.
Whatever you happen to think about the intrinsic worth and predictive ability of psychometrics, real IQ tests are based on math. The math expresses how many other people in their sample population achieved the same raw score that you did, by percentile. Ideally, this sample group is a cross-section of the population that’s representative of the subject’s environment.
The most popular and reliable IQ test for adults is the Wechsler Adult Intelligence Scale, or WAIS, now in its fourth edition. It was normalized against a sample of 2,200 adults. Keep that number in mind.
This graph should be familiar to most people. It’s a Gaussian curve, or normal distribution, of performance on a properly normalized IQ test like the WAIS. Each color change represents one additional standard deviation from the mean.
It’s instructive to look really closely:
Each standard deviation in the WAIS is 15 IQ points further from the mean of 100. Because performance is normalized, only 0.27% of those taking the test are expected to fall outside of 3 standard deviations — in the WAIS this translates to scores above 145 and below 55.
If you take the WAIS and achieve a raw score comparable to only the top 0.135% of the original sample of 2,200, it means your performance is measured relative to 3 people. Score 4 standard deviations above the mean (IQ >= 160) and you’re being compared to just 0.065 other geniuses. In other words, it’s highly likely that no one in the WAIS sample scored as high as you. Congratulations, you are “only” as smart as the lowest estimated IQ of Albert Einstein.
Above (or below) a certain threshold, IQ performance is simply noise. If you extrapolated all the way to IQ 200 (and if you were that smart, you understand why you can’t), you’re scoring a whopping 6 standard deviations above the mean and will have to look elsewhere for your intellectual equals. Since 99.9999998027% of a normally-distributed group falls within 6 standard deviations, the number of members of your uber-Mensa is 6. In the entire world.
The fact is, most adults simply do not know their IQ. Bright children are rarely tested as a matter of course (although some private schools do it). In general, a child is given an IQ test when their school record is lacking, usually because there’s a disparity in expected versus actual performance. IQ testing is a good way to reveal that an otherwise smart kid has a particular learning disability. Psychologists typically do not care whether your IQ is 130 or 145. They want to know if your non-verbal IQ is high but your reading score is below-average; you may have dyslexia and need special educational strategies to succeed.
But the real benefit of IQ is knowing that when someone quotes you a number and it’s greater than 145, it’s safe to assume they’re not as smart as you.