Why isn’t it possible for someone to have zero IQ?
In: Biology
IQ is a normalized measure. You get a bunch of people to take a test, which produces a raw score. Then you find the average of the raw scores. Anybody who scored this average gets assigned an IQ of 100. People who scored 1 standard deviation below the average raw score get an IQ of 85. People who scored 2 standard deviations below get an IQ of 70.
To get an IQ of 0, you would need to score 6 2/3 standard deviations below the mean. (Also, note that a negative IQ is technically possible: 7 standard deviations below the mean should yield an IQ of -5. There’s no sense in which 0 is the absolute bottom for IQ) On a typical test, this simply isn’t possible. Consider a test with a mean raw score of 75 and a standard deviation of 25. The lowest raw score you can get is 0, but that’s only 3 standard deviations below the mean, equating to an IQ of 55. If you really wanted, you could add to the test a lot of questions that are very easy, which would give you the ability to produce a very low IQ if someone happened to get all of them wrong.
But this isn’t really done because it’s unlikely that such a person even exists. If whatever IQ purports to measure (probably not “general intelligence”) is actually normally distributed, we should expect to see someone 6 2/3 standard deviations below the mean in only 1 out of every 100 billion people. That gives pretty low odds of such a person even existing and extremely low odds of you actually testing them.
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