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.