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."