Intelligence of Twins Compared by Science; Physical Traits Studied j The most fascinating phases of twin comparisons are, to the majority major-ity of us, those having to do with intellectual traits, writes D. Cecil Rife in the Scientific American. The relative importance of heredity and environment in mantal make-up has been for centuries, and still is, a topic sure to arouse a lively discussion discus-sion in most groups of civilized mankind. man-kind. Have twin studies shed any light on this question? Considerable research has been done in this field, by numerous investigators. in-vestigators. There are certain difficulties, diffi-culties, however, which are not often encountered in the study of physical traits. We cannot be sure whether a given mental test measures only innate learning capacity or whether part of the responses are the result of education and training. Then, too, the person tested may or may not respond to the best of his ability. There are certain tests, such as the Simon-Binet and its revisions, which have been thoroughly studied by psychologists over a period of years, and have been shown to be fairly valid measures of general learning capacity. Hundreds of twins reared together have been given general intelligoncn tests, by various investigators. The average intra-pair difference obtained ob-tained for identical twins is approximately approxi-mately five points in I.Q. an Insignificant Insig-nificant figure. On the other hand, fraternal twins show, on an average, intra pair differences of from ten to twelve points, which It a significant signifi-cant difference. The obvlc-us conclusion conclu-sion from such results fs that Intelligence In-telligence is, to some degree at leaM, dependent upon genetic make-up.