1.Although twins of two-egg origin resemble each other in intelligence no more than do ordinary siblings, twins of one-egg origin (having presumably identical heredity) are in the vast majority of cases of almost identical intellectual ability. In fact no very large differences have ever been reported for identical twins who have been reared apart.

2.Foster children adopted in the first year of life have been shown by Burks to yield almost a zero correlation with their foster parents in Stanford-Binet intelligence scores. Burks' data indicate that the contribution of heredity to variational differences among children of modern American communities is about five times as great as the contribution of the nurture factors connected with the home. It is true that Freeman's study of foster children was considerably more favorable to environmental influences, but the children he studied were in most cases adopted at later ages, when selective influences could have entered to distort results. Obviously the only way to avoid the selective factor in such a case is to study only foster children who were adopted so early in life that it would not be possible for the more intelligent parents to select the more intelligent children.

3. Children of widely different grades of intelligence are found in exactly the same environment. Siblings, for example, are found to differ over a range from an intelligence quotient of 50 or less to one of 150 or more.

4. Individual differences in intelligence are evidenced from very early years. They are readily detectable by the age of twelve months and at thirty-six months are about as marked as they are at any later age. One would certainly not expect this to be the case if nurture factors were paramount.

5. Experimental attempts to improve the intelligence of backward and defective children have met with extremely-little success. If I knew how to raise the true I.Q. 's of ordinary public school children by 25 points, say from the level of 75 I.Q- (extreme dullness) to 100 I.Q. (average intelligence), I could be a millionaire within ten years. I do not know how, nor does anyone. One can increase the found I.Q. in a given intelligence to this extent by specific coaching on the test questions, but this is a spurious improvement; on other tests and in learning ability the child remains practically what he was before.

6.Finally, the fact that white rats show marked family resemblances in ability to learn mazes and to solve other problems requiring intelligence argues for the hereditary nature of intellectual differences generally. The actuality of family resemblances in the case of rats has been decisively demonstrated both at Stanford University and the University of California. These resemblances certainly cannot be explained as due to cultural influences!

The inheritance of special abilities has been less extensively investigated for the reason that not many kinds of special ability have thus far been successfully isolated and measured. By means of the Seashore tests it has been demonstrated that the elemental factors of musical ability show a very marked tendency to run in families and to be but slightly improvable by tuition. There is much genealogical evidence that the same is probably true of artistic and scientific ability, but owing to the difficulty of ruling out nurture factors the evidence from this source is not conclusive. A child's mechanical ability has been found not to be greatly influenced, as a rule, by the mechanical environment in the home.

About the inheritance of character and personality traits still less is known. Dr. May's tests of trustworthiness have shown nearly as great resemblance between siblings in proneness to cheat, lie, and deceive as others have found for intelligence; but the possibility must be freely admitted that such correlations may be explained as merely reflecting the moral atmosphere of the home. However, F. A. Woods finds that in the royal families which he studied, moral traits characteristic of a certain stock often appear very persistently among branches of the family that have been entirely separated from each other for a generation or more. There is evidence enough that tendencies to psychopathic disorders are transmitted by inheritance. It is probable that important differences in temperament are innately determined, perhaps as correlates of inherited glandular traits. Kretchmer and others have attempted, not entirely without success, to relate certain personality types to types of body-build. Herrington has found reliable, though fairly low, correlations between introversion-extraversion test scores and various chemical reactions of the body; and Rich has found chemical correlates of emotional excitability. No one doubts that the easily recognizable temperamental differences among various breeds of dogs, horses, and other domestic animals are, for the most part, innate.

When we come to such personality traits as fair-mindedness, radicalism, conservatism, or mental masculinity and femininity, the nurture factors loom much larger. These traits are the product of a great variety of attitudes and interests which seem to be readily moulded in early life. The study of mental masculinity and femininity, by means of a test specially devised for the purpose, shows (1) that one's rating in this trait bears almost no relation to masculinity or femininity of body build; (2) that family resemblances in the trait are low; and (3) that extreme inversion in the trait can often be accounted for by environmental factors. Omitting from consideration cases of complete or partial hermaphroditism, which are extremely rare, it appears that the vast majority of persons with inverted sexual inclinations (that is, homosexuals) are made so by their early experiences. Parent fixations are common causal factors. As from 2 to 5 per cent of the general adult population show more or less marked homosexuality, either overt or latent, it is evident that the problem is much more serious than it is usually supposed to be.