THE INHERITANCE OF MENTAL CAPACITIES
We have come to the climax of the eugenist's preliminary argument; if
the main differences between human beings are not due to anything in the
environment or training, either of this or previous generations, there
can be but one explanation for them.
They must be due to the ancestry of the individual--that is, they must
be matters of heredity in the ordinary sense, coupled with the
fortuitous variations which accompany heredity throughout the organic
world.
We need not limit ourselves, however, to the argument by exclusion, for
it is not difficult to present direct evidence that the differences
between men are actually inherited by children from parents. The
problem, formally stated, is to measure the amount by which the likeness
of individuals of like ancestry surpasses the likeness of individuals of
different ancestry. After subtraction of the necessary amount for the
greater likeness in training, that the individuals of like ancestry will
have, whatever amount is left will necessarily, represent the actual
inheritance of the child from its ancestors--parents, grandparents, and
so on.
Obviously, the subtraction for environmental effects is the point at
which a mistake is most probable. We may safely start, therefore, with a
problem in which no subtraction whatever need be made for this cause.
Eye color is a stock example, and a good one, for it is not conceivable
that home environment or training would cause a change in the color of
brothers' eyes.
The correlation[30] between brothers, or sisters, or brothers and
sisters--briefly, the fraternal resemblance--for eye-color was found by
Karl Pearson, using the method described in Chapter I, to be .52. We are
in no danger of contradiction if we state with positiveness that this
figure represents the influence of ancestry, or direct inheritance, in
respect to this particular trait.
Suppose the resemblance between brothers be measured for stature--it
is .51; for cephalic index, that is, the ratio of width of skull to length
of skull--it is .49; for hair color--it is .59. In all of these points,
it will be admitted that no home training, or any other influence except
heredity, can conceivably play an important part. We could go on with a
long list of such measurements, which biometrists have made; and if they
were all summed up it would be found that the fraternal correlation in
these traits as to the heritability of which there can be no dispute, is
about .52. Here is a good measure, albeit a technical one, of the
influence of heredity from the near ancestry. It is possible, too, to
measure the direct correlation between a trait in parent and the same
trait in offspring; the average of many cases where only heredity can be
thought to have had any effect in producing the result, is .49. By the
two methods of measurement, therefore, quite comparable results are
obtained.
So much work has been done in this subject that we have no hesitation in
affirming .5 to represent approximately the average intensity of
heredity for physical characters in man. If any well-marked physical
character be measured, in which training and environment can not be
assumed to have had any part, it will be found, in a large enough number
of subjects, that the resemblance, measured on a scale from 0 to 1, is
just about one-half of unity. Of course, perfect identity with the
parents is not to be expected, because the child must inherit from both
parents, who in turn each inherited from two parents, and so on.
So far, it may be said, we have had plain sailing because we have
carefully chosen traits in which we were not obliged to make any
subtraction whatever for the influence of training. But it is evident
that not all traits fall in that class.
This is the point at which the inheritance of mental traits has been
most often questioned. Probably no one will care to dispute the
inheritance of such physical traits as eye-color. But in considering the
mind, a certain school of popular pseudo-psychological writers question
the reality of mental inheritance, and allege that the proofs which the
geneticist offers are worthless because they do not make account of the
similarity in environment or training. Of course, it is admitted that
some sort of a mental groundwork must be inherited, but extremists
allege that this is little more than a clean slate on which the
environment, particularly during the early years of childhood, writes
its autograph.
We must grant that the analysis of the inheritance of mental traits is
proceeding slowly. This is not the fault of the geneticist, but rather
of the psychologist, who has not yet been able to furnish the geneticist
with the description of definite traits of such a character as to make
possible the exhaustive analysis of their individual inheritance. That
department of psychology is only now being formed.
We might even admit that no inherited "unit character" in the mind has
yet been isolated; but it would be a great mistake to assume from this
admission that proof of the inheritance of mental qualities, in general,
is lacking.
The psychologists and educators who think so appear either to be swayed
by metaphysical views of the mind, or else to believe that resemblance
between parent and offspring is the only evidence of inheritance that
can be offered. The father dislikes cheese, the son dislikes cheese.
"Aha, you think that that is the inheritance of a dislike for cheese,"
cries the critic, "but we will teach you better." An interesting example
of this sort of teaching is furnished by Boris Sidis, whose feelings are
outraged because geneticists have represented that some forms of
insanity are hereditary. He declaims for several pages[31] in this
fashion:
"The so-called scientific method of the eugenists is radically faulty,
in spite of the rich display of colored plates, stained tables,
glittering biological speculations, brilliant mathematical formulæ and
complicated statistical calculations. The eugenists pile Ossa on Pelion
of facts by the simple method of enumeration which Bacon and the
thinkers coming after him have long ago condemned as puerile and futile.
From the savage's belief in sympathetic, imitative magic with its
consequent superstitions, omens, and taboos down to the articles of
faith and dogmas of the eugenists we find the same faulty, primitive
thought, guided by the puerile, imbecile method of simple enumeration,
and controlled by the wisdom of the logical _post hoc, ergo propter
hoc_."
Now if resemblance between parent and offspring were, as Dr. Sidis
supposes, the only evidence of inheritance of mental traits which the
eugenist can produce, his case would indeed be weak. And it is perfectly
true that "evidence" of this kind has sometimes been advanced as
sufficient by geneticists who should have known better. But this is not
the real evidence which genetics offers. The evidence is of numerous
kinds, and several lines might be destroyed without impairing the
validity of the remainder. It is impossible to review the whole body of
evidence here, but some of the various kinds may be indicated, and
samples given, even though this involves the necessity of repeating some
things we have said in earlier chapters. The reader will then be able to
form his own opinion as to whether the geneticists' proofs or the mere
assurances of those who have not studied the subject are the more
weighty.
1. _The analogy from breeding experiments._ Tame rats, for instance, are
very docile; their offspring can be handled without a bit of trouble.
The wild rat, on the other hand, is not at all docile.
W. E. Castle, of Harvard University, writes:[32] "We have repeatedly
mated tame female rats with wild males, the mothers being removed to
isolated cages before the birth of the young. These young which had
never seen or been near their father were very wild in disposition in
every case. The observations of Yerkes on such rats raised by us
indicates that their wildness was not quite as extreme as that of the
pure wild rat but closely approached it."
Who can suggest any plausible explanation of their conduct, save that
they inherited a certain temperament from their sire? Yet the
inheritance of temperament is one of the things which certain
psychologists most "view with alarm." If it is proved in other animals,
can it be considered wholly impossible in man?
2. _The segregation of mental traits._ When an insane, or epileptic, or
feeble-minded person mates with a normal individual, in whose family no
taint is found, the offspring (generally speaking) will be mentally
sound, even though one parent is not. On the other hand, if two people
from tainted stocks marry, although neither one may be personally
defective, part of their offspring will be affected.
This production of sound children from an unsound parent, in the first
case, and unsound children from two apparently sound parents in the
second case, is exactly the opposite of what one would expect if the
child gets his unsoundness merely by imitation or "contagion." The
difference can not reasonably be explained by any difference in
environment or external stimuli. Heredity offers a satisfactory
explanation, for some forms of feeble-mindedness and epilepsy, and some
of the diseases known as insanity, behave as recessives and segregate in
just the way mentioned. There are abundant analogies in the inheritance
of other traits in man, lower animals and plants, that behave in exactly
the same manner.
If mental defects are inherited, then it is worth while investigating
whether mental excellencies may not also be.
3. _The persistence of like qualities regardless of difference in
environment._ Any parent with open eyes must see this in his own
children--must see that they retained the inherited traits even when
they left home and lived under entirely different surroundings. But the
histories of twins furnish the most graphic evidence. Galton, who
collected detailed histories of thirty-five pairs of twins who were
closely alike at birth, and examined their history in after years,
writes:[33] "In some cases the resemblance of body and mind had
continued unaltered up to old age, notwithstanding very different
conditions of life;" in other cases where some dissimilarity developed,
it could be traced to the influence of an illness. Making due allowance
for the influence of illness, yet "instances do exist of an apparently
thorough similarity of nature, in which such differences of external
circumstances as may be consistent with the ordinary conditions of the
same social rank and country do not create dissimilarity. Positive
evidence, such as this, can not be outweighed by any amount of negative
evidence."
Frederick Adams Woods has brought forward[34] a piece of more exact
evidence under this head. It is known from many quantitative studies
that in physical heredity, the influence of the paternal grandparents
and the influence of the maternal grandparents is equal; on the average
one pair will contribute no more to the grandchildren than the other. If
mental qualities are due rather to early surroundings than to actual
inheritance, this equality of grandparental influence is incredible in
the royal families where Dr. Woods got his material; for the grandchild
has been brought up at the court of the paternal grandfather, where he
ought to have gotten all his "acquirements," and has perhaps never even
seen his maternal grandparents, who therefore could not be expected to
impress their mental peculiarities on him by "contagion." When Dr. Woods
actually measured the extent of resemblance to the two sets of
grandparents, for mental and moral qualities, he found it to be the same
in each case; as is inevitable if they are inherited, but as is
incomprehensible if heredity is not largely responsible for one's mental
make-up.
4. _Persistence of unlike qualities regardless of sameness in the_
_environment._ This is the converse of the preceding proposition, but
even more convincing. In the last paragraph but one, we mentioned
Galton's study (cited at some length in our Chapter I) of "identical"
twins, who are so much alike at birth for the very good reason that they
have identical heredity. This heredity was found to be not modified,
either in the body or the mind, by ordinary differences of training and
environment. Some of Galton's histories[35] of ordinary, non-identical
twins were also given in Chapter I; two more follow:
One parent says: "They have been treated exactly alike; both were
brought up by hand; they have been under the same nurse and governess
from their birth, and they are very fond of each other. Their increasing
dissimilarity must be ascribed to a natural difference of mind and
character, as there has been nothing in their treatment to account for
it."
Another writes: "This case is, I should think, somewhat remarkable for
dissimilarity in physique as well as for strong contrast in character.
They have been unlike in mind and body throughout their lives. Both were
reared in a country house and both were at the same schools until the
age of 16."
In the face of such examples, can anyone maintain that differences in
mental make-up are wholly due to different influences during childhood,
and not at all to differences in germinal make-up? It is not necessary
to depend, under this head, on mere descriptions, for accurate
measurements are available to demonstrate the point. If the environment
creates the mental nature, then ordinary brothers, not more than four or
five years apart in age, ought to be about as closely similar to each
other as identical twins are to each other; for the family influences in
each case are practically the same. Professor Thorndike, by careful
mental tests, showed[36] that this is not true. The ordinary brothers
come from different egg-cells, and, as is known from studies on lower
animals, they do not get exactly the same inheritance from their
parents; they show, therefore, considerable differences in their psychic
natures. Real identical twins, being two halves of the same egg-cell,
have the same heredity, and their natures are therefore much more
nearly identical.
Again, if the mind is molded during the "plastic years of childhood,"
children ought to become more alike, the longer they are together. Twins
who were unlike at birth ought to resemble each other more closely at 14
than they did at 9, since they have been for five additional years
subjected to this supposedly potent but very mystical "molding force."
Here again Professor Thorndike's exact measurements explode the fallacy.
They are actually, measurably, less alike at the older age; their inborn
natures are developing along predestined lines, with little regard to
the identity of their surroundings. Heredity accounts easily for these
facts, but they cannot be squared with the idea that mental differences
are the products solely of early training.
5. _Differential rates of increase in qualities subject to much
training._ If the mind is formed by training, then brothers ought to be
more alike in qualities which have been subject to little or no
training. Professor Thorndike's measurements on this point show the
reverse to be true. The likeness of various traits is determined by
heredity, and brothers may be more unlike in traits which have been
subjected to a large and equal amount of training. Twins were found to
be less alike in their ability at addition and multiplication, in which
the schools had been training them for some years, than they were in
ability to mark off the A's on a printed sheet, or to write the
opposites to a list of words--feats which they had probably never before
tried to do.
This same proposition may be put on a broader basis.[37] "In so far as
the differences in achievement found amongst a group of men are due to
the differences in the quantity and quality of training which they had
had in the function in question, the provision of equal amounts of the
same sort of training for all individuals in the group should act to
reduce the differences." "If the addition of equal amounts of practice
does not reduce the differences found amongst men, those differences can
not well be explained to any large extent by supposing them to have been
due to corresponding differences in amount of previous practice. If,
that is, inequalities in achievement are not reduced by equalizing
practice, they can not well have been caused by inequalities in previous
practice. If differences in opportunity cause the differences men
display, making opportunity more nearly equal for all, by adding equal
amounts to it in each case should make the differences less.
"The facts found are rather startling. Equalizing practice seems to
increase differences. The superior man seems to have got his present
superiority by his own nature rather than by superior advantages of the
past, since, during a period of equal advantage for all, he increases
his lead." This point has been tested by such simple devices as mental
multiplication, addition, marking A's on a printed sheet of capitals and
the like; all the contestants made some gain in efficiency, but those
who were superior at the start were proportionately farther ahead than
ever at the end. This is what the geneticist would expect, but fits very
ill with some popular pseudo-science which denies that any child is
mentally limited by nature.
6. _Direct measurement of the amount of resemblance of mental traits in
brothers and sisters._ It is manifestly impossible to assume that early
training, or parental behavior, or anything of the sort, can have
influenced very markedly the child's eye color, or the length of his
forearm, or the ratio of the breadth of his head to its length. A
measure of the amount of resemblance between two brothers in such traits
may very confidently be said to represent the influence of heredity; one
can feel no doubt that the child inherits his eye-color and other
physical traits of that kind from his parents. It will be recalled that
the resemblance, measured on a scale from 0 to 1, has been found to be
about 0.5.
Karl Pearson measured the resemblance between brothers and sisters in
mental traits--for example, temper, conscientiousness, introspection,
vivacity--and found it on the average to have the same intensity--that
is, about 0.5. Starch gets similar results in studying school grades.
Professor Pearson writes:[38]
"It has been suggested that this resemblance in the psychological
characters is compounded of two factors, inheritance on the one hand and
training and environment on the other. If so, one must admit that
inheritance and environment make up the resemblance in the physical
characters. Now these two sorts of resemblance being of the same
intensity, either the environmental influence is the same in both cases
or it is not. If it is the same, we are forced to the conclusion that it
is insensible, for it can not influence eye-color. If it is not the
same, then it would be a most marvelous thing that with varying degrees
of inheritance, some mysterious force always modifies the extent of home
influence, until the resemblance of brothers and sisters is brought
sensibly up to the same intensity! Occam's razor[39] will enable us at
once to cut off such a theory. We are forced, I think, literally forced,
to the general conclusion that the physical and psychical characters in
man are inherited within broad lines in the same manner, and with
approximate intensity. The average parental influence is in itself
largely a result of the heritage of the stock and not an extraneous and
additional factor causing the resemblance between children from the same
home."
A paragraph from Edgar Schuster[40] may appropriately be added. "After
considering the published evidence a word must be said of facts which
most people may collect for themselves. They are difficult to record,
but are perhaps more convincing than any quantity of statistics. If one
knows well several members of a family, one is bound to see in them
likenesses with regard to mental traits, both large and small, which
may sometimes be accounted for by example on the one hand or unconscious
imitation on the other, but are often quite inexplicable on any other
theory than heredity. It is difficult to understand how the inheritance
of mental capacity can be denied by those whose eyes are open and whose
minds are open too."
Strictly speaking, it is of course true that man inherits nothing more
than the capacity of making mental acquirements. But this general
capacity is made up of many separate capacities, all of these capacities
are variable, and the variations are inherited. Such seems to us to be
the unmistakable verdict of the evidence.
Our conclusions as to the inheritance of all sorts of mental capacity
are not based on the mere presence of the same trait in parent and
child. As the psychological analysis of individual traits proceeds, it
will be possible to proceed further with the study of the inheritance of
these traits. Some work has been done on spelling, which is particularly
interesting because most people, without reflection, would take it for
granted that a child's spelling ability depends almost wholly on his
training. Professor Thorndike's exposition[41] of the investigation is
as follows:
"E. L. Earle ('03) measured the spelling abilities of some 800 children
in the St. Xavier school in New York by careful tests. As the children
in this school commonly enter at a very early age, and as the staff and
methods of teaching remain very constant, we have in the case of the 180
pairs of brothers and sisters included in the 600 children closely
similar school training. Mr. Earle measured the ability of any
individual by his deviation from the average for his grade and sex, and
found the coefficient of correlation between children of the same family
to be .50. That is, any individual is on the average 50% as much above
or below the average for his age and sex as his brother or sister.
"Similarities of home training might account for this, but any one
experienced in teaching will hesitate to attribute much efficacy to
such similarities. Bad spellers remain bad spellers though their
teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of
spelling ability ('97) found little or no relationship between good
spelling and any one of the popular methods, and little or none between
poor spelling and foreign parentage. Cornman's more careful study of
spelling ('07) supports the view that ability to spell is little
influenced by such differences in school or home training as commonly
exist."
This is a very clear-cut case of a definite intellectual ability,
differences in which might be supposed to be due almost wholly to the
child's training, but which seem, on investigation, to be largely due to
heredity.
The problem may be examined in still greater detail. Does a man merely
inherit manual skill, let us say, or does he inherit the precise kind of
manual skill needed to make a surgeon but not the kind that would be
useful to a watchmaker? Is a man born merely with a generalized
"artistic" ability, or is it one adapted solely for, let us say, music;
or further, is it adapted solely for violin playing, not for the piano?
Galton, in his pioneer studies, sought for data on this question. In
regard to English judges, he wrote: "Do the judges often have sons who
succeed in the same career, where success would have been impossible if
they had not been gifted with the special qualities of their fathers?
Out of the 286 judges, more than _one in every nine_ of them have been
either father, son or brother to another judge, and the other high legal
relationships have been even more numerous. There can not, then, remain
a doubt but that the peculiar type of ability that is necessary to a
judge is often transmitted by descent."
Unfortunately, we can not feel quite as free from doubt on the point as
Galton did. The judicial mind, if that be the main qualification for a
judge, might be inherited, or it might be the result of training. Such a
case, standing alone, is inconclusive.
Galton similarly showed that the sons of statesmen tended to be
statesmen, and that the same was true in families of great commanders,
literary men, poets and divines. In his list of eminent painters, all
the relatives mentioned are painters save four, two of whom were gifted
in sculpture, one in music and one in embroidery. As to musicians,
Mendelssohn and Meyerbeer are the only ones in his list whose eminent
kinsmen achieved their success in other careers than music.
Havelock Ellis, who likewise studied British men of genius, throws
additional light on the subject. "Painters and sculptors," he found,
"constitute a group which appears to be of very distinct interest from
the point of view of occupational heredity. In social origin, it may be
noted, the group differs strikingly in constitution from the general
body of men of genius in which the upper class is almost or quite
predominant. Of 63 painters and sculptors of definitely known origin,
only two can be placed in the aristocratic division. Of the remainder 7
are the sons of artists, 22 the sons of craftsmen, leaving only 32 for
all other occupations, which are mainly of lower middle class character,
and in many cases trades that are very closely allied to crafts. Even,
however, when we omit the trades as well as the cases in which the
fathers were artists, we find a very notable predominance of craftsmen
in the parentage of painters, to such an extent indeed that while
craftsmen only constitute 9.2% among the fathers of our eminent persons
generally, they constitute nearly 35% among the fathers of the painters
and sculptors. It is difficult to avoid the conclusion that there is a
real connection between the father's aptitude for craftsmanship and the
son's aptitude for art.
"To suppose that environment adequately accounts for this relationship
is an inadmissible theory. The association between the craft of builder,
carpenter, tanner, jeweller, watchmaker, woodcarver, ropemaker, etc.,
and the painter's art is small at best, and in most cases is
non-existent."
Arreat, investigating the heredity of 200 eminent European painters,
reached results similar to those of Ellis, according to the latter's
citation.
Arithmetical ability seems similarly to be subdivided, according to Miss
Cobb.[42] She made measurements of the efficiency with which children
and their parents could do problems in addition, subtraction,
multiplication and division, and could copy a column of figures. "The
measurements made," she writes, "show that if, for instance, a child is
much quicker than the average in subtraction, but not in addition,
multiplication or division, it is to be expected that one at least of
his parents shows a like trait; or if he falls below the average in
subtraction and multiplication, and exceeds it in addition and division,
again the same will hold true of at least one of his parents." These
various kinds of arithmetic appear to be due to different functions of
the brain, and are therefore probably inherited independently, if they
are inherited at all.
To assume that the resemblance between parent and offspring in
arithmetical ability is due to association, training and imitation is
not plausible. If this were the case, a class of children ought to come
to resemble their teacher, but they do not. Moreover, the child
sometimes resembles more closely the parent with whom he has been less
associated in daily life.
From such data as these, we conclude that mental inheritance is
considerably specialized. This conclusion is in accord with Burris'
finding (cited by Thorndike) that the ability to do well in some one
high school study is nearly or quite as much due to ancestry as is the
ability to do well in the course as a whole.
To sum up, we have reason to believe not only that one's mental
character is due largely to heredity, but that the details of it may be
equally due to heredity, in the sense that for any particular trait or
complex in the child there is likely to be found a similar trait or
complex in the ancestry. Such a conclusion should not be pushed to the
point of assuming inheritance of all sorts of dispositions that might be
due to early training; on the other hand, a survey of the whole field
would probably justify us in concluding that any given trait is _more
likely than not_ to be inherited. The effect of training in the
formation of the child's mental character is certainly much less than is
popularly supposed; and even for the traits that are most due to
training, it must never be forgotten that there are inherited mental
bases.
If the reader has accepted the facts presented in this chapter, and our
inferences from the facts, he will admit that mental differences between
men are at bottom due to heredity, just as physical differences are;
that they are apparently inherited in the same manner and in
approximately the same degree.