From time to time reference is made in a popular way to a superstition that if twins are of opposite sexes, the female is sterile. How unfounded this belief is in the case of human twins appears from an old investigation of Simpson in Edinburgh. He investigated the family history of 123 married women born twin to males, of whom 112 had families and eleven had none. The proportion of childless marriages was not greater than in the general population. It may be that the mistaken notion is derived by analogy from what is known to be the case among cattle. There the facts are different. In about nine tenths of different-sexed twins the female is sterile. Such females among cattle are commonly known as freemartins. The phenomenon is not known to occur among other species.
Lillie1 of the University of Chicago has furnished a highly probable explanation of