Working memory and processing speed mediate the effect of age on a general ability construct: evidence from the Italian WAIS-IV standardization sample

We used structural equation modeling to study individual age-related differences in working memory, processing speed, and the general ability of intelligence. The study took advantage of the Italian Wechsler Adult Intelligence Scale 4th edition (WAIS-IV) standardization sample (N=2174) to obtain separate measures of working memory, processing speed, and general ability. Core and supplemental subtests were administered to 16–90 years old participants to ensure breadth of cognitive assessment and broad age coverage. Both linear and quadratic ages were associated with processing speed and working memory, which entirely accounted for the relationship between participants' age and general ability. Specific indirect relationships revealed that age-related differences in working memory weighed more than cognitive speed in predicting individual differences in general ability. Processing speed and working memory sharply decreased after 34 and 44 years of age, respectively. Like working memory, general ability decreased after 44 years of age. Bearing in mind the cognitive changes across adulthood and old age, our findings support the view that working memory is a more proximal source of age-related differences in intelligence than the cognitive processing speed.