, put it with Cole Porter flair: Infants have their infancy; adults, adultery. Dr. Barash, who wrote “The Myth of Monogamy” with his psychiatrist-wife, Judith Eve Lipton, cited a scene from the movie “Heartburn” in which a Nora Ephronesque character complains to her father about her husband's philanderings and the father quips that if she'd wanted fidelity, she should have married a swan. Fat lot of good that would have done her, Dr. Barash said: we now know that swans can cheat, too. Instead, the heroine might have considered union with Diplozoon paradoxum, a flatworm that lives in gills of freshwater fish. “Males and females meet each other as adolescents, and their bodies literally fuse together, whereupon they remain faithful until death,” Dr. Barash said. “That's the only species I know of in which there seems to be 100 percent monogamy.” And where the only hearts burned belong to the unlucky host fish.