The relative effect of nature and nurture influences on twin differences.

The data employed in this study consist of a portion of the material gathered for an extensive investigation of identical and fraternal twins. The measurements were made by Professor H. H. Newman (Department of Zoology), Professer F. N. Freeman, Mrs. Bryan Mitchell, and the writer, all of the University of Chicago. A number of articles and monographs based on these data are now in preparation. Identical (monozygotic) twins are those resulting from the fertilization of one egg by a single sperm and from such origin may be considered as having the same heredity. Fraternal (dizygotic) twins are those from two ova fertilized by separate sperms—the embryos developing side by side, each with its own placenta and chorin. For the latter type of twins the heredities will be different. Professor Newman, who is an eminent authority on the biology of twinning, was chiefly interested in the separation of twins into these two classes. A large number of criteria were used by him in classifying twins according to these two types, a full report of which will appear later. The final number of identical twin pairs secured was fifty, while the number of fraternal (like-sexed) twin pairs obtained was fifty-two. These numbers, of course, do not correspond to the proportions of the two types in the general population, but the data were increased to get samples of comparable size.