The two standard deviation for determining high or low IQ is arbitrary and dependent on context
Take IQ testing as an example for how The cutoffs in the bell-shaped curves which psychology often relies on to define normality are determined by context. Two standard deviations below the mean of 100 put you at 70 and predicts the likelihood of school and life difficulties. Two standard deviations above normal put you at 130 and predicts academic and career success. But there isnât any reason to think that having a IQ of 70 is really different from having one of 71 or even 75.9. There is a 5-point error of measurement in the test, many factors may have interfered with optimal test taking, and some people perform in life much better or worse than you might expect just from their IQ.
Selecting 70 as the unique cutoff for clearly disordered intellectual ability is purely an arbitrary convenience that has no particular significance other than that it selects for the lowest 2.5 percent of the population. These individuals are likely to qualify for special services and dispensations that are denied their near and almost identical neighbors. But there is nothing sacred about the two standard deviations below 100 cutoff at IQ 70âit doesnât have a real world meaning. Slightly higher or slightly lower cutoffs would make equal or more sense, depending on the situation. If more resources are available, services should be offered to those with higher IQs than 70. In some environments, people with an IQ of 70 do just fine. And who says that two standard deviations should be the cutoff? Why not one or three or one and a half? The choice is always arbitrary and driven by context, not statistics.
This gets lost in translation. A recent galling example followed the Supreme Court ruling that it is unconstitutional to execute anyone who is suffering from mental retardation. Life versus death now depends on the silly, artificial non-distinction of having an IQ of 70, rather than 71.10
References
- Frances, Allen. (2013). Saving Normal CHAPTER 1. Whatâs Normal and Whatâs Not? (p. 31). New York, NY: HarperCollins.
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