Ever since Darwin, the extravagance in animal displays has intrigued and perplexed human observers. The elaborate mating rituals, lavish decorative displays, complex songs, calls, and dances, raise fascinating questions. In The Handicap Principle, Amotz and Avishag Zahavi offer a unifying theory that brilliantly explains many previously baffling aspects of animal signalling and holds up a mirror in which ordinary human behaviors take on surprising new significance.

Essentially, the theory asserts that for animal signals to be effective they must be reliable, and to be reliable they must impose a cost, or handicap, on the signaler. When a gazelle sights a wolf, for instance, and jumps high into the air several times before fleeing, it is signaling, in a reliable way, that it is in tip-top condition, easily able to outrun the wolf. A human parallel occurs in children's games of tag, where faster children will often taunt their pursuer before running.

By momentarily handicapping itself - expending precious time and energy in this display - the animal underscores the truthfulness of its signal. Such signaling, the authors suggest, serves the interests of both predator and prey, sparing each the exhaustion of a pointless chase.

Perhaps the book's most important application of the Handicap Principle is to the evolutionary enigma of animal altruism. The authors convincingly demonstrate that when one animal helps another, it handicaps itself - assumes a risk or endures a sacrifice - not primarily to benefit its kin or social group but to increase its own prestige within the group and thus signal its status as a partner or rival.

Finally, the Zahavis discuss the working of the Handicap Principle in human social life, touching upon subjects as diverse as body features, the evolution of art, verbal language versus nonverbal communication, and the role of sex in testing the social bond. Homosexuality, suicidal tendencies, and human altruistic drives are all explained within the framework of evolution.