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[Truncated abstract] Central to the study of general intelligence is Spearman’s g, one definition of which is general fluid reasoning ability. At a cognitive level of explanation, it has recently been hypothesized that individual differences in g is based in the executive functions of the frontal lobes. This theory can be contrasted with the theory that the fount of g is speed of information processing. The aim of this thesis was to test the idea that the two contrasting theories of g could be reconciled by invoking an alternative theory which suggests that there may be two g’s one related to individual differences in intelligence and attributable to differences in speed of information processing and one related to the development of intelligence and based in executive functions of the frontal lobes. This was done with a series of neuropsychological studies that tested groups of adults and children with and without putative central nervous system damage on tests of fluid intelligence, executive function, goal-neglect, and speed of information processing. In study 1, three adults with focal frontal lobe lesions and ten adults with frontotemporal dementia (FTD) were administered three common intelligence tests. In comparison to premorbid and crystallized measures on intelligence, the majority of frontal patients exhibited impaired fluid g. In study 2, 10 patients with FTD, 8 patients with Alzheimer?s disease (AD), 10 adult controls, and 15 adults with low fluid g, were tested on a fast and slow version of a goal-neglect task (thought to measure executive functioning) and a measure of speed of processing. A classical double dissociation was found. Frontal patients with impaired fluid g displayed goal-neglect but intact speed of information processing whereas the adults with low fluid g exhibited slowed speed of information 2 processing but not goal-neglect. It was concluded that the link between fluid g and goal-neglect in adults (demonstrated by previous r, Thesis (M.Psych./Ph.D.)--University of Western Australia, 2005