An identical twin's enthusiastic report on what it's all about: the myths, legends, theories, anecdotes, problems, pleasures, and scientific studies. Cassill relates the familiar tales of twins worshipped or killed in older cultures, of rival (Cain & Abel, Jacob & Esau) or deified (Apollo & Diana) twins in legend. A certain uneasiness, or ambivalence, has always attended twin or multiple births, and Cassill discusses this in relation to parental difficulties, hostilities, sibling rivalries, and the mutual closeness (or mutual hostility) between twins themselves. She also takes up newer ideas: the possibility that mirror-image twins (right- and left-handed, or even with reversed organ positions) are twins who just escaped being Siamese, that many singletons are really the survivors of twin pregnancies during which, somehow, the other twin vanished (engulfed? absorbed?). The latter notion brings some talk, too, of patients in psychotherapy who seem to be seeking such a lost partner. (R. D. Laing theorizes similarly in his new Voice of Experience.) Though Cassill considers fraternal twins (along with the varieties possible in multiple births), her chief concern is identicals: the various attempts to trace likenesses and differences on the basis of exactly when the single ovum divided, whether a placenta was shared, etc. She reviews Scheinfeld's and Kallmann's classic twin studies, as well as the recent Minnesota revelations. In general, she goes along with the concordance noted among twins for intelligence and certain mental illnesses as indicative of a hereditary basis--but notes that concordance for personality traits has been harder to demonstrate. She has high praise for twin registries and what twin studies can contribute to research in chronogenetics (the timing, in a life-span, of disease or death). For twins and their families, the book provides reassurance that the intrinsic problems can be surmounted. For the general public, it's currently tops for general information--with a good bibliography for further guidance.