The Distribution of Mental Ability:

...

>The “retarded” category is traditionally described as IQ below 70, but this is rather arbitrary. Psychologists, educators, and social workers, however, show little disagree­ment that the vast majority of persons with IQs below 70 have unusual difficulties in school and generally have difficulties of an intellectual nature as adults. There are few jobs in a modern industrial society for which persons below IQ 70 are capable without making allowances for their intellectual disability or restructuring the usual requirements of the job so as to bring it within the capabilities of the retarded person. Persons below IQ 70 have difficulty in managing their financial affairs and other ordinary demands involv­ing arithmetic and reading comprehension. Many school systems set the retarded classification at IQ 75 or even 80, because such children so often require special instruction to learn the basic school subjects. Even then these children find it very difficult to keep pace with their classmates, because, with a mental age a year or more below the average of their classmates, they lack the conceptual readiness for the scholastic subjects that are taught in a given grade. The armed forces exclude most persons who score below an equivalent of about IQ 75 or 80 on the Armed Forces Qualification Test. There are too few useful occupations that these low-IQ recruits can be successfully trained to perform, with limited time for training.

The American Association of Mental Deficiency has recommended that the border­line of mental retardation be set at between IQ 70 to 85, defining as “ subnormal " IQ deviations of more than one standard deviation below the general mean of the population. But this is a matter of statistical definition and does not agree with the general practice of basing classification as retarded not only on IQ but on various criteria of the individual’s social adjustment and adaptive behavior. The majority of adults with IQs between 70 and 85 are not retarded by ordinary criteria of social adjustment. In one large study, for example, it was found that 84 percent of such persons had completed at least eight years of school, 83 percent had held a job, 65 percent had a semiskilled or higher occupation, 80 percent were financially independent or a housewife, and almost 100 percent were able to do their own shopping and travel alone (Mercer, 1972a).

The AAMD further subdivides the retarded classification as follows:

IQ 85-70                Borderline retardation

IQ 69-55                Mild retardation

IQ 54-40                Moderate retardation

IQ 39-25                Severe retardation

IQ 24 and below   Profound retardation 

The vast majority, some 70 percent to 80 percent, of persons below IQ 70 are biologically normal; no brain damage, disease, or genetic defect is detectable. They are apart of normal variation in the combination of polygenic and environmental factors that contribute to variance in IQ. The remaining 20 percent to 30 percent show “ clinical” signs, either due to brain damage from injury or disease, or they show the medically recognizable signs of one or another of the more than eighty specifically identifiable syndromes associated with a single defective gene or chromosomal anomaly. The bulk of such individuals are found in the IQ range below 50 and nearly all persons with IQs below 35 or 40 are the “ clinical” type.

All levels of retardation, except perhaps some of those at the “profound” level, are amenable to various forms of therapy, conditioning, or training that can make their social behavior and their lives more satisfactory, both to themselves and to those who must take care of them, although such training has no appreciable effect on the IQ. The mildly or educably retarded can benefit from schooling when it is properly geared to their level of readiness and conceptual capabilities.

Moving up the IQ scale to the 80 to 90 range, which is traditionally but unfortu­nately labeled "dull normal", the typical picture is that of quite normal children or adults whose only consistently distinguishing feature is greater than average difficulty in the more academic school subjects. Such children are generally slower to “catch on” to whatever is being taught if it involves symbolic, abstract, or conceptual subject matter. In the early grades in school they most often have problems in reading and arithmetic and are sometimes labeled “slow learners.” But it is really not that they learn slowly as that they lag behind in developmental readiness to grasp the concepts that are within easy reach of the majority of their age mates. Such children will eventually grasp these basic subjects fairly easily, but about a year or two later than their age mates. They are better thought of as “ slow developers” than as “ slow learners. ” The child with an IQ of 80 to 90, it seems, has to be explicitly taught more of what he must learn, in or out of school, than the brighter child, who picks up much more knowledge and skills on his own, without need of direct intervention by parents or teachers. The lower-IQ child does not as readily absorb as much from his own experiences as the brighter child. Most low-IQ children do not eventually catch up with their age mates. Because of the fairly high degree of constancy of the IQ (...), low-IQ children differ increasingly from their average age mates in mental age as they advance in school. Children in the IQ range of 80 to 90 are usually not noticed to be intellectually different in any way until they enter school. Their scholas­tic problems become more evident with each higher grade, since they are further behind their classmates in mental age.

Because of this increasing lag in the academic subjects, most of these children, when they reach junior or senior high school, elect the less academic courses. The elective system allows them to choose courses in which they are more apt to succeed and from which they are more apt to profit in the world of work when they leave school. By high school age many such children, because of their earlier scholastic difficulties, have already acquired a definite dislike for the academic subjects and tend to drop out of high school if it has little else to offer besides the traditional curriculum. Algebra and geometry, foreign languages, English literature, and physics and chemistry are not these pupils’ forte; unless strenuously pressured by parents, they usually avoid these tra­ditionally college preparatory courses. They prefer the more practical and vocational courses, which are less symbolic and abstract. Not surprisingly, as adults they have few if any intellectual interests to speak of; they are generally poorly informed on world affairs, science, the arts, or other types of information gained through reading. They are usually employed in jobs that depend little on scholastic skills or in which advancement depends on individual study or training courses involving “book learning,” “examinations," or other school-like requirements.

The 50 percent of the population classed as “average” and falling between IQs 90 and 110 hardly needs description. They differ from those below and above on the IQ scale only in degree. The schools, the world of work, and the entertainment industry are largely geared to this average majority of the population. Persons with IQs closer to 110, because of their usually more favorable experiences in elementary and high school are more likely to seek advanced training or even college than those with IQs closer to 90. The myriad of occupations below the highly technical and professional levels are mostly occupied by persons in this IQ range. There is virtually no limitation on such persons in fields that are not dependent on university- or graduate-level education or in occupations that require special talents, such as athletics, art, acting, and musical performance. At least an average IQ is usually needed to compete successfully in such fields, but, beyond that, a high level in special talents and personal qualities are the crucial factors in success.

IQs between 110 and 130 are typically found in children who do well in school subjects, who catch on easily to what is being taught at their grade level, and who take a liking to the academic curriculum. They are typically good readers from an early age, they enjoy reading, and they do more of it than most children without encouragement from others. There are great individual differences in interests and special abilities in this group, and these differences are reflected in the variability of these children’s perfor­mances in various scholastic subjects. But usually children with IQs above 115 or so can perform outstandingly in any school subject to which they may apply themselves, barring special disabilities such as aphasia and dyslexia. These above-average children show a wider range of interests than the average, they tend to be self-learners, and their hobbies are usually more complex, advanced, well planned, and long term than one sees in their more average age mates. Colleges and universities obtain their students almost entirely from the range above IQ 110. Entering freshmen in most selective colleges average in the 115 to 120 range, and graduates from these colleges average about 120 to 125. The vast majority of persons in skilled, managerial, and professional occupations are from this group. Manual or “ blue-collar” workers in this IQ range generally become the master carpenters, skilled mechanics, technicians, foremen, and contractors with earnings on a par with or exceeding those of many white-collar and professional workers.

Persons with IQs above 130 usually find school easy, or even perhaps boring for lack of intellectual challenge. Some are not very enthusiastic students until they reach college. With average interest, motivation, and application, they tend to be in the upper half of their college class in grade-point average. With better than average effort and persistence, they can succeed in virtually any occupation except those requiring special talents, among which I would include mathematics; at an advanced level mathematics seems to require not only a high level of general intelligence but also something more in the nature of a special talent. (As in high-level talent for musical composition and in playing chess, there is also a marked sex difference in high-level mathematical ability. Also, it is interesting that authentic child prodigies are found only in chess, music, and mathematics.) Beyond IQ 130, factors other than general intelligence largely account for what these persons make of their careers. Personality factors, interests, drive, stability, perseverance, general health, cultural background, educational opportunities, and special talents become the main determining factors once a high level of general intelligence is present. Outstanding achievement, as Galton noted, depends on at least three things: exceptional general mental ability, exceptional drive, and exceptional perseverance. These qualities are almost invariably illustrated, for example, in the biographies of per­sons whose achievements are judged sufficiently outstanding to be included in the Ency­clopaedia Britannica.<
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Bias in Mental Testing -> p. 109-112
Arthur R. Jensen (1980)

[If you read this excerpt with interest, I highly recommend you to read the whole chapter (or at least p. 105-114). Perhaps I shouldn't have cut it into pieces.]