Geneticist (You and Heredity, etc.) Scheinfeld has written about twins for them--for their parents--in the first complete popular book extending coverage beyond their handling (Betty Gehman's--1965) to include the scientific knowns from conception on. The chances of twinning improve with older mothers, for example, while identical twins face greater prenatal dangers. All twins present problems (should they share the same room at home or the same class at school?) and usually the answers are as dualistic as they are. Even though there may be a greater incidence of retardation among twins, their mental performance once established as normal is just as satisfactory; so is their health in spite of the ""dual threat diseases""; etc. etc. down through analytical treatment, the fictional and scientific literature about them. Scheinfeld is a sound investigator-evaluator and his book should be or real value to all those lucky mothers of bonus babies.