<em>g</em> Presumes Unidimensionality

 In a multidimensional set of interrelations among tests, one axis can be found that accounts for as much of the interrelatedness as possible, even when it is known that more dimensions are required. The g-men have defined that largest dimension as g. They haven’t discovered it, as they are fond of saying, any more than the Greenwich Meridian was discovered by the International Meridian Conference in 1884. Any set of interrelated tests has to have a largest dimension, so under this definition the existence of g is no longer a matter of empirical dispute. Rather, it has simply been defined into existence. But it has no special status. Defining multidimensional ability with a big g-factor and some number of smaller sub-factors is just one out of an infinite number of ways that ability could be aligned along dimensions.

So let’s return to Flynn. He thinks that g used to hold together, as long as our focus was on relations among tests at a single point of time, and has only come apart once he started to examine differential changes in the components of ability over time. But the coherence of g was an illusion, founded on the false intuition that positivity of relations among ability tests was sufficient evidence of unidimensionality, In fact, pace Gottfredson, it would be possible to define separate ability domains for abstract thinking and practical knowledge within a single time point, and these traits would then correspond closely to the courses of generational change that interest Flynn. Such traits would not be the correct way to divide up ability, any more than g is. They would be a plausible solution in a domain where a certain amount of indeterminacy is part of the scientific landscape, and they would be a convenient tool for studying the Flynn effect. In the same way, g is useful for many things, especially for broad-stroke prediction of outcomes like job performance. The trick is not to get hooked on any particular way of dividing up the pie, because it is a short step from there to trying to find the Greenwich Meridian at the bottom of the North Atlantic.

Eric Turkheimer, The Fundamental Intuition (November 11, 2007)

• iq
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