David Wechsler on cognitive capacity

David Wechsler on cognitive capacity

I recently obtained a copy of David Wechsler’s 1935, “The Range of Human Capacities.”  To my delight, Chapter 2, “The Measurement of Human Capacities” is an extended analysis of the concept of capacity as applied to intelligence.

In fact I disagree with his conclusion, and I think the chapter marks an important turning point in the history of intelligence and IQ testing.  He says,

“The general meaning of the word ‘capacity’ is that of cubical contents, and in this sense it is synonymous with ‘volume’.” He goes on, “Thus, cranial capacity refers to the volume of that portion of the skull which contains the brain; vital capacity to the maximum volume of air which a person is able to take into his lungs; and similarly in a few other cases. But more often the original meaning of the term is only indirectly suggested.  Thus, anthropologists may speak of body weight as a physical capacity.”

Unfortunately, this last usage elides a crucial sense of the word “capacity,” most evident in the reference to lung capacity: as a maximum of what something can contain. The etymology of the word (based on five minutes of research on Google) comes from the Latin capax, “that can contain.” If we say that a car’s trunk has a capacity of 15 cubic feet, we are talking about what the trunk can hold, what it has the potential to hold, not how many suitcases are in it right now.

So when Wechsler accepts that, in psychology, “the term capacity has, to all practical purposes, become synonymous [which Wechsler spells synonomous.  Has the spelling of that word changed?] with the word ability” he is making a commitment to an important direction in IQ measurement, which is that IQ is a measure of a person’s ability right now, how many mental suitcases one has in the cranium, not to how big the cranium is, how many suitcases it might hold if it were packed right up to the brim.

By and large I agree with that assessment, although per the last post it is complicated.  The problem is that referring to an IQ score as a capacity implies that it is a measure of potential, of how much intelligence a person can hold.  The use of the word has tempted us to think of IQ as more than it necessarily is.  I am still seeking a good example of IQ as capacity, an instance where a tester can identify someone with poor function and assert that despite appearances they have the potential to do much better.  I don’t necessarily doubt that such exists, I just want to see a good example of it.

Eric Turkheimer

Eric Turkheimer is the Project Leader for the Genetics and Human Agency Project. Eric is a clinical psychologist and behavioral geneticist. For thirty years he has been involved in empirical and theoretical investigations of the implications of genetics for the genesis of complex human behavior. Current projects include understanding the interaction between socioeconomic status and the heritability of intelligence, and philosophical analysis of the ethical status of work that purports to demonstrate biologically based differences in behavior among racial groups.

  • James Thompson
    Posted at 17:13h, 08 November Reply

    One might infer greater ability than that measured if there were some barriers to performance, including inattention, fatigue etc. However, the imputed greater capacity must be evident when the person is tested after rest and in favourable circumstances. Usually, peak performance is judged the true measure of capacity, though that should be tempered by a correction for guessing.

  • Eric Turkheimer
    Posted at 21:57h, 08 November Reply

    That’s interesting, but you are referring to a short-term capacity– the best one could do right now, if not tired or distracted or whatever. I guess I am thinking about a longer-term capacity, for example how someone might be able to perform if they had been better educated, or if they lived in a home that promoted cognitive ability. So a child is doing badly in the first grade, behind in reading and math. She is given an IQ test, which says she has an IQ of 85. Does this explain why she is performing badly in school– she has low cognitive capacity– or is it just an alternate description of it? Is it possible to use intelligence tests to discriminate between children who could improve with greater attention (they are not at capacity) and those for whom additional efforts would fail (because they are already at capacity)?

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