Ms. Peck, who founded the National Organization for Non-Parents and inveighed against parenting in The Baby Trap (1971), now speaks up for the one-child family, armed with more impregnable facts and cozy generalizations. Lindbergh was an only child and Leonardo and Baudelaire. So are such notables as Rex Reed and Sammy Davis, Jr. Only children are high achievers in disproportionate numbers, she maintains; they develop exceptional language skills early, have a richer intellectual potential, rebel less vehemently against adult values, and are healthier physically and mentally. Properly handled (""It is necessary to plan for playmates""), they can avoid loneliness and grow up free of those negative traits like selfishness commonly ascribed to them. What's more, sibling rivalry has been overrated as a socializing force. Much of Peck's argument is based on research studies; in addition, her own survey questions have been included here. For the most part she explores valid areas (""do most adult sibling ties tend to be close?"") although some of the questions are imprecisely phrased (""Have you usually felt pretty sure your life would work out the way you want it to?"") and at times the significance of the results has been inflated. But she goes on so relentlessly that even those sympathetic to her point of view will question her insistence on its absolute superiority. In discounting the truisms about large families she barely admits that siblings can matter to each other; she focuses on the added work they mean for parents (""One is fun. . . more's a chore""), virtually overlooking the added pleasures. And she sees only a positive edge to a boy's protestation, ""I've got my dad to play with, and he's more interesting than a little kid."" Popular appeal despite the single-track perspective.