Inspection time correlates with general speed of processing but not with fluid ability

Inspection time (IT) has been shown to correlate reliably with putative tests of general intelligence such as Raven's matrices and more strongly with performance IQ than with verbal IQ from the Wechsler scales. A common interpretation of this pattern has been that PIQ and matrices tests are measures of fluid ability (Gf) and therefore that IT provides an index of a biological substrate underpinning individual differences in IQ. Marker tests for live of the constructs described in Gf-Gc theory of cognitive abilities were administered to a sample of 64 adults who also completed IT estimation. IT correlated -0.43 (p < 0.001) with a test of general processing speed (Gs) but did not significantly correlate with a test of Gf Results of partial correlation analysis and factor analysis were not consistent with the proposition that general intelligence depends exclusively or substantially on speed of processing. A sub-sample of 37 subjects completed a second marker test for both Gf and Gs. The correlations with IT were consistent with those obtained for the larger sample. The results are interpreted within a hierarchical model of cognitive ability that incorporates speed of processing at the same level as fluid ability and crystallized ability.