There are two attempts to define intelligence that represent agreement among large numbers of psychometricians.The oldest of these is documented in the 1988 text The IQ Controversy:The Media and Public Policy by Snyderman and Rothman (of Harvard and Smith College, respectively).It reports the results of a large-scale query of 661 scholars who were selected as a random sample of members of academic and other professional organizations.They were given detailed questionnaires concerning intelligence testing, but were not asked to answer questions outside their areas of competence.These scholars reported strong agreement on the topics that were later reported by Murray and Herrnstein and others.The scholars were asked what attributes they considered to be important elements of intelligence. 99.3% mentioned the ability to reason abstractly; 97.7% listed problem solving; and 96% listed the ability to learn.The authors concluded: On the whole, respondents seem to believe that intelligence tests are doing a good job measuring intelligence, as they would define it.

More recently, Linda Gottfredson composed the following: Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience.It is not merely book learning, a narrow academic skill, or test-taking smarts.Rather, it reflects a broader and deeper capability for comprehending our surroundings

catching on,

making sense of things, or

figuring out, what to do. This comment appeared in a letter titled Mainstream Science on Intelligence, published on the editorial page of the Wall Street Journal, Tuesday, December 13, 1994.It was signed by 52 psychometricians who are known worldwide for their publications in peer-reviewed journals.

IQ derives almost all of its validity because it is a good proxy for psychometric g.Arthur Jensen reports (P. 91, The g Factor) a g loading of about 0.88 for most IQ tests.Jensen suggests that the word intelligence not be used in scientific discussions, because it lacks a scientific definition, and that we should instead focus on g, since it is unambiguously defined as the product of a hierarchical factor analysis.It happens that all categories of test items correlate with one another to at least some degree.The ultimate relationship between the various categories of cognitive activity is reflected as g, which is common to all mental abilities.Anyone interested in how g is defined and derived, should consult The g Factor, 1998.

Having narrowed intelligence down to whatever it is that is reflected in g, the next logical question is what is it that causes variations in g? Charles Spearman discovered g in the early 1900s and developed factor analysis, plus a number of statistical methods that are used in psychometrics and other fields.Curiously, Spearman was never satisfied that he understood the nature of g.Today, g remains somewhat difficult to comprehend, but there is now much evidence that it is based in physiology.We know that g is substantially, and possibly entirely, heritable.We also know that there are some identified genes; one example is Igf2r, which is associated with high intelligence.Although there are various physiological candidates that may account for some degree of what we see as g, two are especially salient:nerve conduction velocity (NCV) and myelination.

NCV correlates to g, with faster NCV in more intelligent brains.This is an area that has been extensively studied and reported by Arthur Jensen.While there may be various ways in which NCV affects mental activity, the one that Jensen most often mentions relates to the high volatility of information held in working memory.Faster movement of the information, implies that it can be used before it is lost.

Mensan Ed Miller noted that Jensen s explanation of the role of NCV in intelligence does not explain the observation that the standard deviation of response times to external stimuli correlates negatively with IQ (smaller SD reflects higher IQ).Miller s paper, Intelligence and Brain Myelination:A Hypothesis, first appeared in Personality and Individual Differences, Vol. 17 (December 1994), and was later reprinted in the Mensa Research Journal.By assembling a huge number of observations, Miller has been able to support his hypothesis that variations in intelligence is a function of the degree of myelination in the brain.His explanation for this is very lengthy, but is basically that myelin acts as an insulator preventing neural noise from disrupting brain signals in a manner that is somewhat like cross-talk that is seen in electrical circuits.As noise disrupts brain signals, the brain attempts to retransmit the information.If there is a significant loss of transmission integrity, there is a cascading of errors, overwhelming the brain so that it cannot progress.This is apparently what is seen in testing of people by increasingly difficult problems.A point is reached in which the person cannot solve the problem in any amount of time.They simply cannot manage the number of bits of information that must be manipulated for a solution.

Another strong indication that intelligence stems from physiology is that g can be measured passively with results that are as good as standard power IQ tests.This can be done by two quite different methods.One method is known as electroencephalography, or EEG.This method involves a variety of measures which are taken from averaging a large number of brain waves.The test subject need only be connected to the measurement equipment his brain waves are evoked by a clicking sound.Measurements are based on such things as the complexity (string length) of the average signals, and the zero-crossing points of specific parts of the typical response pattern.More intelligent brains produce a more complex form (greater string length).The technique is a completely passive and involuntary observation of brain activity.

The second method, consists of a battery of very simple tests, known as Elementary Cognitive Tests (ECT).The test subject is asked to press a button when he observes a simple condition, such as the turning on of lights, or the playing of a sound, or the movement of a line on a projection screen.All such tests can be done by virtually all subjects in less than one second.Some of the tests involve comparisons or discriminations, but are not difficult to perform, even by very dull people.Sensors on the scalp measure the brain reaction time for each task.More intelligent subjects have shorter average brain reaction times (this does not include hand movement time).Likewise the standard deviations of higher IQ subjects is lower than for less intelligent subjects.Interestingly, the reaction time correlation is independent from the standard deviation correlation, strongly implying that they have different causes.Each ECT is somewhat g loaded, but when combined, the net measurement correlates to test results by standard IQ tests as closely as one IQ test correlates to another one.

The importance of EEG and ECT testing may extend into many aspects of understanding and measuring intelligence, but the most significant aspect of these modern laboratory techniques is that they seem to be more direct ways of getting to the physiological nature of intelligence.Both techniques seem to give support to Miller s myelination model, and both indicate that it is communication between widely separated regions of that brain that is central to the variation we observe in intelligence.As something of a side note, both EEG and ECT measurements show the same variations between racial groups as are seen with traditional IQ tests.

Among the other interesting aspects of physiological correlates to intelligence, more intelligent brains show the following:

¨                   PET scans show that they are more efficient with respect to glucose uptake.

¨                   MRI imaging shows higher gray-white matter contrast.

¨                   Brain volume is greater (correlation of +.40, after correcting for body size).

¨                   In MRI examinations, there is a phenomenon known as T2 relaxation time, which is shorter.The importance of this is that the T2 relaxation time is an indication of the number of biological membranes in the immediate vicinity of the affected protons (in water).

Less earth shaking, but interesting correlations (all from The g Factor):

¨                  Highly intelligent people have 15-20% more allergies than their immediate family members.

¨                  There are somewhat more highly intelligent and retarded people who are left handed.

¨                  IQ correlates with vital capacity, hand grip strength, symmetrical facial features, light hair color, light eye color, above average basic metabolic rate, inability to taste certain chemicals.(Mostly small correlations.)

¨                 Even after factoring out SES, the correlation between general health and g is +.326.

I should not omit that Mensan Miles Storfer has published a good bit about the relationship between myopia and intelligence.One good starting point for his observations is his book Intelligence and Giftedness.

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