Editing G factor (psychometrics) (section)

==Theories==
{{See also|Two-factor theory of intelligence|g-VPR model|Parieto-frontal integration theory|Neural efficiency hypothesis}}
While the existence of ''g'' as a statistical regularity is well-established and uncontroversial among experts, there is no consensus as to what causes the positive intercorrelations. Several explanations have been proposed.<ref name="Hunt 2011, 94">Hunt 2011, 94</ref>

===Mental energy or efficiency===

Charles Spearman reasoned that correlations between tests reflected the influence of a common causal factor, a general mental ability that enters into performance on all kinds of mental tasks. However, he thought that the best indicators of ''g'' were those tests that reflected what he called ''the eduction of relations and correlates'', which included abilities such as [[Deductive reasoning|deduction]], [[Inductive reasoning|induction]], problem solving, grasping relationships, inferring rules, and spotting differences and similarities. Spearman hypothesized that ''g'' was equivalent with "mental energy". However, this was more of a metaphorical explanation, and he remained agnostic about the physical basis of this energy, expecting that future research would uncover the exact physiological nature of ''g''.<ref>Jensen 1998, 18–19, 35–36, 38. The idea of a general, unitary mental ability was introduced to psychology by [[Herbert Spencer]] and [[Francis Galton]] in the latter half of the 19th century, but their work was largely speculative, with little empirical basis.</ref>

Following Spearman, [[Arthur Jensen]] maintained that all mental tasks tap into ''g'' to some degree. According to Jensen, the ''g'' factor represents a "distillate" of scores on different tests rather than a summation or an average of such scores, with factor analysis acting as the [[distillation]] procedure.<ref name="Jensen 2002"/> He argued that ''g'' cannot be described in terms of the item characteristics or information content of tests, pointing out that very dissimilar mental tasks may have nearly equal ''g'' loadings.  Wechsler similarly contended that ''g'' is not an ability at all but rather some general property of the brain. Jensen hypothesized that ''g'' corresponds to individual differences in the speed or efficiency of the neural processes associated with mental abilities.<ref>Jensen 1998, 91–92, 95</ref> He also suggested that given the associations between ''g'' and [[#Elementary cognitive tasks|elementary cognitive tasks]], it should be possible to construct a [[ratio scale]] test of ''g'' that uses [[time]] as the unit of measurement.<ref>Jensen 2000</ref>

===Sampling theory===

The so-called sampling theory of ''g'', originally developed by [[Edward Thorndike]] and [[Godfrey Thomson]], proposes that the existence of the positive manifold can be explained without reference to a unitary underlying capacity. According to this theory, there are a number of uncorrelated mental processes, and all tests draw upon different samples of these processes. The inter correlations between tests are caused by an overlap between processes tapped by the tests.<ref>Mackintosh 2011, 157</ref><ref>Jensen 1998, 117</ref> Thus, the positive manifold arises due to a measurement problem, an inability to measure more fine-grained, presumably uncorrelated mental processes.<ref name="maas"/>

It has been shown that it is not possible to distinguish statistically between Spearman's model of ''g'' and the sampling model; both are equally able to account for inter correlations among tests.<ref>Bartholomew et al. 2009</ref> The sampling theory is also consistent with the observation that more complex mental tasks have higher ''g'' loading, because more complex tasks are expected to involve a larger sampling of neural elements and therefore have more of them in common with other tasks.<ref>Jensen 1998, 120</ref>

Some researchers have argued that the sampling model invalidates ''g'' as a psychological concept, because the model suggests that ''g'' factors derived from different test batteries simply reflect the shared elements of the particular tests contained in each battery rather than a ''g'' that is common to all tests. Similarly, high correlations between different batteries could be due to them measuring the same set of abilities rather than ''the'' same ability.<ref name="horn&mcardle"/>

Critics have argued that the sampling theory is incongruent with certain empirical findings. Based on the sampling theory, one might expect that related cognitive tests share many elements and thus be highly correlated. However, some closely related tests, such as forward and backward digit span, are only modestly correlated, while some seemingly completely dissimilar tests, such as vocabulary tests and Raven's matrices, are consistently highly correlated. Another problematic finding is that brain damage frequently leads to specific cognitive impairments rather than a general impairment one might expect based on the sampling theory.<ref name="maas"/><ref>Jensen 1998, 120–121</ref>

===Mutualism===

The "mutualism" model of ''g'' proposes that cognitive processes are initially uncorrelated, but that the positive manifold arises during individual development due to mutual beneficial relations between cognitive processes. Thus there is no single process or capacity underlying the positive correlations between tests. During the course of development, the theory holds, any one particularly efficient process will benefit other processes, with the result that the processes will end up being correlated with one another. Thus similarly high IQs in different persons may stem from quite different initial advantages that they had.<ref name="maas"/><ref>Mackintosh 2011, 157–158</ref> Critics have argued that the observed correlations between the ''g'' loadings and the heritability coefficients of subtests are problematic for the mutualism theory.<ref name="RJ2010">Rushton & Jensen 2010</ref>