I.Q. and real-life functioning

© Paul Cooijmans

Introduction

This is a list of I.Q. ranges with for each a brief description of typical functioning and other features. The I.Q.'s are expressed on a scale with a general population mean of 100 and standard deviation of 15. They refer to scores on adult tests only, by adult norms. The exact cut-offs for the ranges are arbitrary, and one should realize that functioning may depend on more than I.Q. alone.

In addition it is known that I.Q. has the greatest significance to real-life functioning (and the highest correlation with "g", the common factor shared by all mental ability tests) at its lower and average ranges, and becomes less important as one goes higher; the more you have of it, the less important it gets, just as with money. It is unknown whether I.Q.'s beyond about 140 have any extra significance.

Brief overview of the I.Q. ranges

Retarded Below average Average Above average Intelligent Profound Severe Moderate Mild "Borderline retarded"

"Gifted" High Very high Pervasive Exceptional <20 20-34 35-49 50-69 70-79 80-89 90-99 100-109 110-119 120-129 130-139 140-149 150-159 160-169 170-179 180-...

Descriptions of the I.Q. ranges

Lower than 20 — Profound retardation

Usually multi-handicapped with obvious physical deformities and short life expectancy. Heavily dependent on others. Can learn no or only the very simplest tasks.

20-34 — Severely retarded

Basic intellectual tasks, including language, are difficult to learn. Can learn some self-care behaviour but remain dependent on others. There are usually motor problems and physical anomalies. Usually not employable.

Profound and severe retardation are typically caused by brain damage during pregnancy, at birth, or early in life, and as such not genetic and not inherited.

35-49 — Moderately retarded

Can learn simple life skills and employment tasks with special education. May be employed in special settings, and achieve some independence. Often socially immature. Self-awareness — having an inner image of self, realizing that one is a person separate from the others around one — may exist from here on, but is not guaranteed to exist as it depends on more than intelligence alone. The most intelligent non-human animals, such as some crows, chimpanzees, bonobos, parrots, and dolphins, are in this range. Bonobo or chimpanzee I.Q. scores are sometimes even quoted as high as 80 or 90, but those are childhood age-peer scores that correspond to adult I.Q.'s of only just over 40.

50-69 — Mildly retarded

Educable, can learn to care for oneself, employable in routinized jobs but require supervision. Might live alone but do best in supervised settings. Immature but with adequate social adjustment, usually no obvious physical anomalies.

Moderate and mild retardation, contrary to the more severe forms, are typically not caused by brain damage but part of the normal variance of intelligence, and therefore largely genetic and inherited. This is important with regard to the question whether or not retarded persons should have children; for especially the moderate and mild forms of retardation, with which it is physically possible to have children, are the most likely to be inherited.

70-79 — Borderline retarded

Limited trainability. Have difficulty with everyday demands like using a phone book, reading bus or train schedules, banking, filling out forms, using appliances like a video recorder, microwave oven, or computer, et cetera, and therefore require assistance from relatives or social workers in the management of their affairs. Can be employed in simple tasks but require supervision.

Above the threshold for normal independent functioning. Can perform explicit routinized hands-on tasks without supervision as long as there are no moments of choice and it is always clear what has to be done. Assembler, food service.

This is also the I.Q. range most associated with violence. Most violent crime is committed by males from this range. This does not imply that all males in this range are violent, nor that all violent males are in this range. But when the modal I.Q. of a group is in this range, one may expect trouble with with many male members of that group. When the modal I.Q. of a society or population is raised upward of this range, violence decreases as fewer males fall in this range then, given the shape of an even remotely normal distribution. When the modal I.Q. of a society is below this range to begin with though, raising it may increase violence. The causal mechanism behind the (statistical) relation between crime and below-average I.Q. is likely that lower I.Q. levels inherently tend to go with having less impulse control, being less able to delay gratification, being less able to comprehend moral principles like the Golden Rule, and being overstrained by the cognitive demands of society.

And, this is the range into which men of average or just above average intelligence sink when under the influence of alcohol; alcohol reduces I.Q. by up to about 25 points while drunk (own data), which explains why many drunk men are violent and aggressive (own hypothesis).

90-99 — Average

Able to learn a trade in a hands-on manner and perform tasks involving decisions. Craftsman, sales, police officer, clerk. Studies involving some theory are possible from this range upward.

100-109 — Average

Able to learn from written materials. Employable in senior positions.

110-119 — Above average

Able to learn in "college" format. Bachelor degrees. Manager, teacher, accountant. Just capable of taking high-range I.Q. tests.

120-129 — Above average

Capable of gathering and inferring own information. Master degrees. Attorney, chemist, executive. About 93 % of high-range test candidates score I.Q. 120 or higher.

130-139 — "Gifted"

May just be able to write a legible piece of text like an article or modest novel. Minor literary figures. Ph.D. in the "soft" sciences. In this range lies the mode of scores on high-range tests, and almost 80 % of high-range candidates score I.Q. 130 or higher. Regular psychology's I.Q. tests should not be trusted beyond this range as their validity breaks down here, if such scores are given at all.

140-149 — Intelligent

Capable of rational communication and scientific work. From this range on, only specific high-range tests should be considered. Important scientific discoveries and advancement are possible from the upper part of this range on.

We do not know if intelligence from about this range on is simply the extreme end of a normal distribution centred at 100 and largely formed by heredity, or if high intelligence in some cases has other causes (non-inherited or non-genetic) which make it deviate from the normal curve centred at 100 and form a "bump" in the far right tail, similar to the bump in the retarded range (which has non-inherited and non-genetic causes). And since we possess no physical, absolute scale of intelligence, these questions are hitherto meaningless altogether.

About one in two high-range test candidates score I.Q. 140 or higher.

150-159 — Highly intelligent

About one in four high-range test candidates score I.Q. 150 or higher. Otherwise under investigation.

160-169 — Very highly intelligent

About one in ten high-range test candidates score I.Q. 160 or higher. Otherwise under investigation.

170-179 — Pervasively intelligent

About one in a hundred high-range test candidates score I.Q. 170 or higher. Otherwise under investigation; a report on this specific group is Statistics of the top scorers.

180-185 — Exceptionally intelligent

In this range one would expect the I.Q.'s of the few most intelligent individuals alive. About one in a thousand high-range test candidates score I.Q. 180 or higher.

Colophon

The information in this article is based on a combination of sources, such as various publicly available descriptions of forms of retardation, various scientific and popular books about intelligence, and personal observation in contact with several thousands of people whose I.Q. scores on a wide array of tests are known; the latter concerns over 2000 persons with well over 5000 scores on several hundred different tests.