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Simple reaction time measures a very simple situation - how quickly somebody can respond by pressing a button (or equivalent) to a sudden-onset stimulus such as a light flashing or a sharp noise.
This correlates with general intelligence - very robustly, but not with a very high correlation.
In other words, the sRT-g positive correlation is found in all populations - but there is a lot of scatter around the correlation line; so there are apparently many individuals with slow sRT and high IQ and vice versa.
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(I would suppose that when there is a dissociation between sRT and g, that in principle this should be explicable; for example I would suppose that a person with rapid sRT but low measured IQ would have some factor, such as poor vision, concentration or motor control, which affected their performance in IQ testing - but not {or hardly at all} performance in reaction time testing.)
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The advantage of measuring the sRT (compared with other ways of measuring intelligence) is that it is highly objective (e.g. culture- and education-independent); and also that it reduces to a low level of influence things like perceptual differences (good eyesight and hearing), concentration (this does not need to be sustained for sRT), and 'motor' differences - such as muscle strength and fine motor control.
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(Many people grossly misunderstand reaction times as being analogous to reactive power sports actions such as boxing prowess, or batting in baseball; but the sports most related to fast sRT are finesse, minimal-movement, quick-reaction but not strength-dependent sports such as Ping-Pong/ Table Tennis, or perhaps foil fencing - in other words, the kind of competitive sports at which East Asians tend to excel.)
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The simple reaction time therefore measures the speed of a nerve path through the central nervous system while minimizing the effect of perception and musculature - and this presumably provides an imprecise but valid measure of the processing speed of the CNS.
And this is a measure of the efficiency of processing.
So, I assume that sRT is a measure of CNS efficiency - and this is why sRT correlates with g; because g is perhaps most profoundly a measure of efficiency: and increased CNS efficiency is the reason that increased g improves (to a greater or lesser extent) average performance on all cognitive tests and tasks.
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It is a truism of complex systems research (and for very good reasons) that the main method to increase efficiency is via an increased in complexity - more specifically efficiency in a process usually entails functional specialization and coordination of specialized functions.
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(The best known example is in a factory - such as Adam Smith's pin factory described in Wealth of Nations - by breaking down pin manufacture into multiple specialized functions - division of labour - and careful coordination of these functions, the efficiency of the factory can be greatly increased when measured in terms of daily productive output of pins per employee. )
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By this argument - sRT measures speed of processing, which is a measure of efficiency, which is a measure of complex functional specialization of organization.
So, the sRT correlation with g is telling us that - at root:
A more intelligent brain is a brain with a greater functional complexity.
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(Complexity of the brain is partly a matter of size as a way of accommodating more neural connectedness; but probably also of the density of connectedness. In computer technology, the density of connectedness of circuits is a constraint on processing speed - and probably something similar applies to brains.)
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If a more intelligent brain is more complex (hence more efficient and faster-processing) then this, in turn, explains a number of otherwise puzzling things about general intelligence - for example that higher intelligence in a human group or population is achieved at some cost - such as reduced athleticism and reduced fertility in that population.
Some cost, because building a more complex brain is costly - it requires extra resources (or transfer of resources from other functions) and extra time to make something that is extra complex - and there are more things which can go wrong either in making it, or after it has been made.
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