How Much Can We Boost IQ and Scholastic Achievement? pdf


I was curious to read this article becus of all the bad things ive heard about it. however, it turned out to be not what i expected at all. its a very sensible well-researched well-written article. not at all any racist bigotry. it cud still serve as a reasonable introduction to the science of intelligence.



Occupational Correlates of Intelligence

Intelligence, as we are using the term, has relevance considerably beyond the

scholastic setting. This is so partly because there is an intimate relationship be-

tween a society’s occupational structure and its educational system. Whether we

like it or not, the educational system is one of society’s most powerful mechanisms

for sorting out children to assume different roles in the occupational hierarchy.

The evidence for a hierarchy of occupational prestige and desirability is unam-

biguous. Let us consider three sets of numbers.2

First, the Barr scale of occupations, devised in the early 1920s, provides one set of data. Lists of 120 representative occupations, each definitely and concretely described, were given to 30 psy-

chological judges who were asked to rate the occupations on a scale from o to 100

according to the grade of intelligence each occupation was believed to require for

ordinary success. Second, in 1964, the National Opinion Research Center (NORC),

by taking a large public opinion poll, obtained ratings of the prestige of a great

number of occupations; these prestige ratings represent the average standing of

each occupation relative to all the others in the eyes of the general public.

Third, a rating of socioeconomic status (SES) is provided by the 1960 Census of

Population: Classified Index of Occupations and Industries, which assigns to each

of the hundreds of listed occupations a score ranging from 0 to 96 as a composite

index of the average income and educational level prevailing in the occupation.

The interesting point is the set of correlations among these three independent-

ly derived occupational ratings.

The Barr scale and the NORC ratings are correlated .91.

The Barr scale and the SES index are correlated .81.

The NORC ratings and the SES index are correlated .90.

In other words, psychologists’ concept of the “intelligence demands” of an occu-

pation (Barr scale) is very much like the general public’s concept of the prestige

or “social standing” of an occupation (NORC ratings), and both are closely re-

lated to an independent measure of the educational and economic status of the

persons pursuing an occupation (SES index). As O. D. Duncan (1968, pp. 90-91)

concludes, “. . . ‘intelligence’ is a socially defined quality and this social definition

is not essentially different from that of achievement or status in the occupational

sphere. . . . When psychologists came to propose operational counterparts to the

notion of intelligence, or to devise measures thereof, they wittingly or unwittingly

looked for indicators of capability to function in the system of key roles in the

society.” Duncan goes on to note, “Our argument tends to imply that a correla-

tion between IQ and occupational achievement was more or less built into IQ

tests, by virtue of the psychologists’ implicit acceptance of the social standards

of the general populace. Had the first IQ tests been devised in a hunting culture,

‘general intelligence’ might well have turned out to involve visual acuity and

running speed, rather than vocabulary and symbol manipulation. As it was, the

concept of intelligence arose in a society where high status accrued to occupations

involving the latter in large measure, so that what we now mean by intelligence

is something like the probability of acceptable performance (given the opportu-

nity) in occupations varying in social status.”





Evidence from Studies of Selective Breeding

The many studies of selective breeding in various species of mammals provide

conclusive evidence that many behavioral characteristics, just as most physical

characteristics, can be manipulated by genetic selection (see Fuller & Thompson,

1962; Scott and Fuller, 1965). Rats, for example, have been bred for maze learn-

ing ability in many different laboratories. It makes little difference whether one

refers to this ability as rat “intelligence,” “learning ability” or some other term—

we know that it is possible to breed selectively for whatever the factors are that

make for speed of maze learning. To be sure, individual variation in this com-

plex ability may be due to any combination of a number of characteristics in-

volving sensory acuity, drive level, emotional stability, strength of innate turning

preferences, brain chemistry, brain size, structure of neural connections, speed

of synaptic transmission, or whatever. The point is that the molar behavior of

learning to get through a maze efficiently without making errors (i.e., going

up blind alleys) can be markedly influenced in later generations by selective

breeding of the parent generations of rats who are either fast or slow (“maze

bright” or “maze dull,” to use the prevailing terminology in this research) in

learning to get through the maze. Figure 4 shows the results of one such

genetic selection experiment. They are quite typical; within only six generations

of selection the offspring of the “dull” strain make 100 percent more errors in

learning the maze than do the offspring of the “bright” strain (Thompson,

1954). In most experiments of this type, of course, the behaviors that respond

so dramatically to selection are relatively simple as compared with human in-

telligence, and the experimental selection pressure is severe, so the implications

of such findings for the study of human variation should not be overdrawn.

Yet geneticists seem to express little doubt that many behavioral traits in

humans would respond similarly to genetic selection. Three eminent geneticists

(James F. Crow, James V. Neel, and Curt Stern) of the National Academy of

Sciences recently prepared a “position statement,” which was generally hedged

by extreme caution and understatement, that asserted: “Animal experiments

have shown that almost any trait can be changed by selection. . . . A selection

program to increase human intelligence (or whatever is measured by various

kinds of ‘intelligence’ tests) would almost certainly be successful in some measure.

The same is probably true for other behavioral traits. The rate of increase would

be somewhat unpredictable, but there is little doubt that there would be prog-

ress” (National Academy of Sciences, 1967, p. 893).

pretty inteteresting!



For some human characteristics the degree of assortative mating is effectively

zero. This is true of fingerprint ridges, for example. Men and women are ob-

viously not attracted to one another on the basis of their fingerprints. Height,

however, has an assortative mating coefficient (i.e., the correlation between

mates) of about .30. The IQ, interestingly enough, shows a higher degree of as-

sortative mating in our society than any other measurable human characteristic.

I have surveyed the literature on this point, based on studies in Europe and

North America, and find that the correlation between spouses’ intelligence test

scores averages close to +.6o. Thus, spouses are more alike in intelligence than

brothers and sisters, who are correlated about .50.

As Eckland (1967) has pointed out, this high correlation between marriage

partners does not come about solely because men and women are such excellent

judges of one another’s intelligence, but because mate selection is greatly aided

by the highly visible selective processes of the educational system and the occu-

pational hierarchy. Here is a striking instance of how educational and social

factors can have far-reaching genetic consequences in the population. One

would predict, for example, that in preliterate or preindustrial societies as-

sortative mating with respect to intelligence would be markedly less than it is

in modern industrial societies. The educational screening mechanisms and socio-

economic stratification by which intelligence becomes more readily visible would

not exist, and other traits of more visible importance to the society would take

precedence over intelligence as a basis for assortative mating. Even in our own

society, there may well be differential degrees of assortative mating in different

segments of the population, probably related to their opportunities for educa-

tional and occupational selection. When any large and socially insulated group

is not subject to the social and educational circumstances that lead to a high

degree of assortative mating for intelligence, there should be important genetic

consequences. One possible consequence is some reduction of the group’s ability,

not as individuals but as a group, to compete intellectually. Thus probably one

of the most cogent arguments for society’s promoting full equality of educational,

occupational, and economic opportunity lies in the possible genetic consequences

of these social institutions.


pretty high numbers?! perhaps the effect is less strong now a days?



Effects of Inbreeding on Intelligence

One of the most impressive lines of evidence for the involvement of genetic fac-

tors in intelligence comes from study of the effects of inbreeding, that is, the

mating of relatives. In the case of polygenic characteristics the direction of the

effect of inbreeding is predictable from purely genetic considerations. All in-

dividuals carry in their chromosomes a number of mutant or defective genes.

These genes are almost always recessive, so they have no effect on the phenotype

unless by rare chance they match up with another mutant gene at the same locus

on a homologous chromosome; in other words, the recessive mutant gene at a

given locus must be inherited from both the father and mother in order to affect

the phenotype. Since such mutants are usually defective, they do not enhance

the phenotypic expression of the characteristic but usually degrade it. And for

polygenic characteristics we would expect such mutants to lower the metric value

of the characteristics by graded amounts, depending upon the number of paired

mutant recessives. If the parents are genetically related, there is a greatly in-

creased probability that the mutant recessives at given loci will be paired in the

offspring. The situation is illustrated in Figure 8, which depicts in a simplified

way a pair of homologous chromosomes inherited by an individual from a moth-

er (M) and father (F) who are related (Pair A) and a pair of chromosomes

inherited from unrelated parents (Pair B). The blackened spaces represent reces-

sive genes. Although both pairs contain equal numbers of recessives, more of

them are at the same loci in Pair A than in Pair B. Only the paired genes degrade

the characteristics’ phenotypic value.


A most valuable study of this genetic phenomenon with respect to intelli-

gence was carried out in Japan after World War II by Schuil and Neel (1965).

The study illustrates how strictly sociological factors, such as mate selection, can

have extremely important genetic consequences. In Japan approximately five

percent of all marriages are between cousins. Schuil and Neel studied the

offspring of marriages of first cousins, first cousins once removed, and

second cousins. The parents were statistically matched with a control group of

unrelated parents for age and socioeconomic factors. Children from the cousin

marriages and the control children from unrelated parents (total N = 2,111)

were given the Japanese version of the Wechsler Intelligence Scale for Children

(WISC). The degree of consanguinity represented by the cousin marriages in

this study had the effect of depressing WISC IQs by an average of 7.4 percent,

making the mean of the inbred group nearly 8 IQ points lower than the mean of

the control group. Assuming normal distributions of IQ, the effect is shown in

Figure 9, and illustrates the point that the most drastic consequences of group

mean differences are to be seen in the tails of the distributions. In the same study

a similar depressing effect was found for other polygenic characteristics such as

several anthropometric and dental variables.


The mating of relatives closer than cousins can produce a markedly greater

reduction in offspring’s IQs. Lindzey (1967) has reported that almost half of a

group of children born to so-called nuclear incest matings (brother-sister or

father-daughter) could not be placed for adoption because of mental retarda-

tion and other severe defects which had a relatively low incidence among the

offspring of unrelated parents who were matched with the incestuous parents in

intelligence, socioeconomic status, age, weight, and stature. In any geographi-

cally confined population where social or legal regulations on mating are lax,

where individuals’ paternity is often dubious, and where the proportion of half–

siblings within the same age groups is high, we would expect more inadvertent

inbreeding, with its unfavorable genetic consequences, than in a population in

which these conditions exist to a lesser degree.


surprising that the effects are so large.


consider the effect this has on the world IQ average given data such as:




Abdominal Decompression.

There is now evidence that certain manipulations

of the intrauterine environment can affect the infant’s behavioral development

for many months after birth. A technique known as abdominal decompression

was invented by a professor of obstetrics (Heyns, 1963), originally for the pur-

pose of making women experience less discomfort in the latter months of their

pregnancy and also to facilitate labor and delivery. For about an hour a day

during the last three or four months of pregnancy, the woman is placed in a de-

vice that creates a partial vacuum around her abdomen, which greatly reduces

the intrauterine pressure. The device is used during labor up to the moment of

delivery. Heyns has applied this device to more than 400 women. Their infants,

as compared with control groups who have not received this treatment, show more

rapid development in their first two years and manifest an overall superiority in

tests of perceptual-motor development. They sit up earlier, walk earlier, talk

earlier, and appear generally more precocious than their own siblings or other

children whose mothers were not so treated. At two years of age the children in

Heyns’ experiment had DQs (developmental quotients) some 30 points higher

than the control children (in the general population the mean DQ is 100, with

a standard deviation of 15). Heyns explains the effects of maternal abdominal

decompression on the child’s early development in terms of the reduction of intra-

uterine pressure, which results in a more optimal blood supply to the fetus and

also lessens the chances of brain damage during labor. (The intrauterine pres-

sure on the infant’s head is reduced from about 22 pounds to 8 pounds.) Re-

sults on children’s later IQs have not been published, but correspondence with

Professor Heyns and verbal reports from visitors to his laboratory inform me that

there is no evidence that the IQ of these children is appreciably higher beyond

age 6 than that of control groups. If this observation is confirmed by the proper

methods, it should not be too surprising in view of the negligible correlations

normally found between DQs and later IQs. But since abdominal decompression

results in infant precocity, one may wonder to what extent differences in intra-

uterine pressure are responsible for normal individual and group differences in

infant precocity. Negro infants, for example, are more precocious in develop-

ment (as measured on the Bayley Scales) in their first year or two than Caucasian

infants (Bayley, 1965a). Infant precocity would seem to be associated with more

optimal intrauterine and perinatal conditions. This conjecture is consistent with

the finding that infants whose prenatal and perinatal histories would make them

suspect of some degree of brain damage show lower DQs on the Bayley Scales

than normal infants (Honzik, 1962). Writers who place great emphasis on the

hypothesis of inadequate prenatal care and complications of pregnancy to account

for the lower average IQ of Negroes (e.g., Bronfenbrenner, 1967) are also obliged

to explain why these unfavorable factors do not also depress the DQ below

average in Negro infants, as do such factors as brain damage and prenatal and

infant malnutrition (Cravioto, 1966). Since all such environmental factors

should lower the heritability of intelligence in any segment of the population

in which they are hypothesized to play an especially significant role, one way to

test the hypothesis would be to compare the heritability of intelligence in that

segment of the population for which extra environmental factors are hypothe-

sized with the heritability in other groups for whom environmental factors are

supposedly less accountable for IQ variance.


never heard of this before




The literature on the relationship of premature birth to the child’s

IQ is confusing and conflicting. Guilford (1967), in his recent book on The

Nature of Intelligence, for example, concluded, as did Stoddard (1943), that

prematurity has no effect on intelligence. Stott (1966), on the other hand, pre-

sents impressive evidence of very significant IQ decrements associated with pre-

maturity. Probably the most thorough review of the subject I have found, by

Kushlick (1966), helps to resolve these conflicting opinions. There is little ques-

tion that prematurity has the strongest known relation to brain dysfunction of

any reproductive factor, and many of the complications of pregnancy are strongly

associated with the production of premature children. The crucial factor in pre-

maturity, however, is not prematurity per se, but low birth-weight. Birth-weight

apparently acts as a threshold variable with respect to intellectual impairment.

All studies of birth-weight agree in showing that the incidence of babies weighing

less than 5-1/2 lbs. increases from higher to lower social classes. But only about

1 percent of the total variance of birth-weight is accounted for by socioeconomic

variables. Race (Negro versus white) has an effect on birth-weight independently

of socioeconomic variables. Negro babies mature at a lower birth-weight than

white babies (Naylor & Myrianthopoulos, 1967). If prematurity is defined as a

condition in which birth-weight is under 5-1/2 lbs., the observed relationship

between prematurity and depression of the IQ is due to the common factor of low

social class. Kushlick (1966, p. 143) concludes that it is only among children

having birth-weights under 3 lbs. that the mean IQ is lowered, independently

of social class, and more in boys than in girls. The incidence of extreme subnor-

mality is higher for children with birth-weights under 3 or 4 lbs. But when one

does not count these extreme cases (IQs below 50), the effects of prematurity or

low birth-weight—even as low as 3 lbs.—have a very weak relationship to chil-

dren’s IQs by the time they are of school age. The association between very low

birth-weight and extreme mental subnormality raises the question of whether

the low birth-weight causes the abnormality or whether the abnormality arises

independently and causes the low birth-weight.


Prematurity and low birth-weight have a markedly higher incidence among

Negroes than among whites. That birth-weight differences per se are not a pre-

dominant factor in Negro-white IQ differences, however, is suggested by the find-

ings of a study which compared Negro and white premature children matched for

birth-weight. The Negro children in all weight groups performed significantly

less well on mental tests at 3 and 5 years of age than the white children of com-

parable birth-weight (Hardy, 1965, p. 51).


isnt it rather that blacks hav shorter gestation times relative to whites? not that they get more premature births. well, premature compared to white standards, but thats not a good way of looking at it. one might as well say that whites have many postmature births compared to blacks.




Birth Order.

Order of birth contributes a significant proportion of the variance in

mental ability. On the average, first-born children are superior in almost every

way, mentally and physically. This is the consistent finding of many studies (Altus,

1966) but as yet the phenomenon remains unexplained. (Rimland [1964, pp.

140-143] has put forth some interesting hypotheses to explain the superiority of

the first-born.) Since the first-born effect is found throughout all social classes

in many countries and has shown up in studies over the past 80 years (it was first

noted by Galton), it is probably a biological rather than a social-psychological

phenomenon. It is almost certainly not a genetic effect. (It would tend to make

for slightly lower estimates of heritability based on sibling comparisons.) It is

one of the sources of environmental variance in ability without any significant

postnatal environmental correlates. No way is known for giving later-born chil-

dren the same advantage. The disadvantage of being later-born, however, is very

slight and shows up conspicuously only in the extreme upper tail of the distribu-

tion of achievements. For example, there is a disproportionate number of first-

born individuals whose biographies appear in Who’s Who and in the Encyclope-

dia Britannica.


thats interesting. it immediately comes to mind that birth order has a storng effect on male homosexuality as well. so, this wud mean that male homosexuality and lower g shud be somewhat related.






Since the human brain attains 70 percent of its maximum adult weight

in the first year after birth, it should not be surprising that prenatal and infant

nutrition can have significant effects on brain development. Brain growth is

largely a process of protein synthesis. During the prenatal period and the first

postnatal year the brain normally absorbs large amounts of protein nutrients

and grows at the average rate of 1 to 2 milligrams per minute (Stoch & Smythe,

1963; Cravioto, 1966).


Severe undernutrition before two or three years of age, especially a lack of

proteins and the vitamins and minerals essential for their anabolism, results in

lowered intelligence. Stoch and Smythe (1963) found, for example, that extreme-

ly malnourished South African colored children were some 20 points lower in IQ

than children of similar parents who had not suffered from malnutrition. The

difference between the undernourished group and the control group in DQ and

IQ over the age range from 1 year to 8 years was practically constant. If under-

nutrition takes a toll, it takes it early, as shown by the lower DQs at 1 year and

the absence of any increase in the decrement at later ages. Undernutrition occur-

ring for the first time in older children seems to have no permanent effect. Se-

verely malnourished war prisoners, for example, function intellectually at their

expected level when they are returned to normal living conditions. The study

by Stoch and Smythe, like several others (Cravioto, 1966; Scrimshaw, 1968), also

revealed that the undernourished children had smaller stature and head circum-

ference than the control children. Although there is no correlation between in-

telligence and head circumference in normally nourished children, there is a

positive correlation between these factors in groups whose numbers suffer varying

degrees of undernutrition early in life. Undernutrition also increases the corre-

lation between intelligence and physical stature. These correlations provide us

with an index which could aid the study of IQ deficits due to undernutrition in

selected populations.


this seems to hav been proven wrong. there is a correlation, but its small, about 0.20. see:

Rushton, J. P., & Ankney, C. D. (2009). Whole-brain size and general mental ability: A review. International Journal of Neuroscience, 119, 691-731.



Race Differences

The important distinction between the individual and the population must al-

ways be kept clearly in mind in any discussion of racial differences in mental

abilities or any other behavioral characteristics. Whenever we select a person for

some special educational purpose, whether for special instruction in a grade–

school class for children with learning problems, or for a “gifted” class with an

advanced curriculum, or for college attendance, or for admission to graduate

training or a professional school, we are selecting an individual, and we are se-

lecting him and dealing with him as an individual for reasons of his individual-

ity. Similarly, when we employ someone, or promote someone in his occupation,

or give some special award or honor to someone for his accomplishments, we are

doing this to an individual. The variables of social class, race, and national origin

are correlated so imperfectly with any of the valid criteria on which the above de-

cisions should depend, or, for that matter, with any behavioral characteristic,

that these background factors are irrelevant as a basis for dealing with individuals

—as students, as employees, as neighbors. Furthermore, since, as far as we know,

the full range of human talents is represented in all the major races of man and

in all socioeconomic levels, it is unjust to allow the mere fact of an individual’s

racial or social background to affect the treatment accorded to him. All persons

rightfully must be regarded on the basis of their individual qualities and merits,

and all social, educational, and economic institutions must have built into them

the mechanisms for insuring and maximizing the treatment of persons according

to their individual behavior.


Jensen was an evil bigot, right, right? …



Another aspect of the distribution of IQs in the Negro population is their

lesser variance in comparison to the white distribution. This shows up in most

of the studies reviewed by Shuey. The best single estimate is probably the estimate

based on a large normative study of Stanford-Binet IQs of Negro school chil-

dren in five Southeastern states, by Kennedy, Van De Riet, and White (1963).

They found the SD of Negro children’s IQs to be 12.4, as compared with 16.4 in

the white normative sample. The Negro distribution thus has only about 60 per-

cent as much variance (i.e., SD2) as the white distribution.


There is an increasing realization among students of the psychology of the dis-

advantaged that the discrepancy in their average performance cannot be com-

pletely or directly attributed to discrimination or inequalities in education. It

seems not unreasonable, in view of the fact that intelligence variation has a large

genetic component, to hypothesize that genetic factors may play a part in this

picture. But such an hypothesis is anathema to many social scientists. The idea

that the lower average intelligence and scholastic performance of Negroes

could involve, not only environmental, but also genetic, factors has indeed been

strongly denounced (e.g., Pettigrew, 1964). But it has been neither contradicted

nor discredited by evidence.


The fact that a reasonable hypothesis has not been rigorously proved does not

mean that it should be summarily dismissed. It only means that we need more

appropriate research for putting it to the test. I believe such definitive research

is entirely possible but has not yet been done. So all we are left with are various

lines of evidence, no one of which is definitive alone, but which, viewed all to-

gether, make it a not unreasonable hypothesis that genetic factors are strongly

implicated in the average Negro-white intelligence difference. The preponderance

of the evidence is, in my opinion, less consistent with a strictly environmental

hypothesis than with a genetic hypothesis, which, of course, does not exclude the

influence of environment or its interaction with genetic factors.


1) the smaller SD might be due to bad sampling. if the sample is not representative, that might explain it. if that is the case, one shud observe some more non-normality in the data, probably with a loss of people in the low end, say -1-2 SD, so around 55-70.


2) i think the proponderance of the evidence today is really strong. so strong that a purely environmental theory cannot be rationally held.



It has been argued by Harry and Margaret Harlow that “human beings in our

world today have no more, or little more, than the absolute minimal intellec-

tual endowment necessary for achieving the civilization we know today” (Harlow

& Harlow, 1962, p. 34). They depict where we would probably be if man’s average

genetic endowment for intelligence had never risen above the level corresponding

to IQ 75: “. . . the geniuses would barely exceed our normal or average level;

comparatively few would be equivalent in ability to our average high school

graduates. There would be no individuals with the normal intellectual capacities

essential for making major discoveries, and there could be no civilization as we

know it.”


It may well be true that the kind of ability we now call intelligence was needed

in a certain percentage of the human population for our civilization to have

arisen. But while a small minority—perhaps only one or two percent—of highly

gifted individuals were needed to advance civilization, the vast majority were

able to assimilate the consequences of these advances. It may take a Leibnitz or

a Newton to invent the calculus, but almost any college student can learn it and

use it.


Since intelligence (meaning g) is not the whole of human abilities, there may

be some fallacy and some danger in making it the sine qua non of fitness to play

a productive role in modern society. We should not assume certain ability re-

quirements for a job without establishing these requirements as a fact. How often

do employment tests, Civil Service examinations, the requirement of a high school

89 diploma, and the like, constitute hurdles that are irrelevant to actual perfor-

mance on the job for which they are intended as a screening device? Before going

overboard in deploring the fact that disadvantaged minority groups fail to clear

many of the hurdles that are set up for certain jobs, we should determine whether

the educational and mental test barriers that stand at the entrance to many of

these employment opportunities are actually relevant. They may be relevant only

in the correlational sense that the test predicts success on the job, in which case

we should also know whether the test measures the ability actually required on

the job or measures only characteristics that happen to be correlated with some

third factor which is really essential for job performance. Changing people in

terms of the really essential requirements of a given job may be much more fea-

sible than trying to increase their abstract intelligence or level of performance

in academic subjects so that they can pass irrelevant tests.


good point.





Leave a Reply