Ridley is hardly a newcomer to the debates over evolutionary psychology. An earlier book, The Red Queen (1993), tackled the evolution of sexuality and is one of the most respected treatments of the subject. Expressed by its title, this previous book’s metaphor for evolution is inspired by Lewis Carroll’s Alice in Wonderland, where the Red Queen explains that she must continually keep moving ahead just to stay in the same place. As with the predicament of the Red Queen, evolution involves an arms race between organisms and their enemies, which endlessly transform themselves just to stay competitive. Following other recent biologists, Ridley argues that sexual choices and strategies have evolved to keep pathogens, such as bacteria and viruses, off balance. By continually shuffling their genetic deck of immunological locks, sexual organisms are attempting to foil the pathogens’ latest molecular keys. For example, a pathogen may have evolved the ability to attach itself to a particular cellular receptor, enabling it to penetrate the cell wall and cause illness. Sexual recombination of the host’s genes during reproduction continually changes these molecular locks, blocking the pathogen’s previous method of entry.
The Origins of Virtue takes up where The Red Queen left off, namely, with Homo sapiens. Unafraid of confronting difficult problems, Ridley has devoted his latest book to the paradox that so troubled Darwin: If inherited dispositions, which have evolved by natural selection, are always in the service of the individual, then why is cooperation observed in nature? The book is organized on three different levels: genes, social behavior, and theories about social behavior. Ridley begins his story with genes. In higher organisms, he explains, genes have teamed up as chromosomal “parliaments,” which in turn have teamed up as cells, which subsequently evolved into creatures with specialized organs. But why have genes—which, according to selfish gene theory, care only about reproducing themselves—entered into such complex cellular alliances?
The answer involves Hamilton’s notion of inclusive fitness, along with Adam Smith’s insight into the division of labor. Cells, on this view, are to be seen as “close relatives” because they possess identical genes, a circumstance that encourages them to act altruistically whenever cooperative actions enhance their ability to have descendants. The division of labor facilitates this outcome by allowing specialized cells to do their tasks more efficiently than generalist cells. For example, cells that are specialized for digesting food make more efficient use of available nutrients, and well-fed organisms tend to have more offspring.
Potential conflicts continually arise between genes, even within the same cell, because genes, rather than cells or whole organisms, are the primary replicators of life. These potential conflicts must be suppressed, Ridley explains, in order to keep genetic parliaments working together. Consider pregnancy—usually understood as a wonderfully harmonious relationship between mother and fetus. From a Darwinian perspective, pregnancy is nothing of the kind. The fetus, which has genes from both father and mother, is twice as related to itself as it is to its mother, so it is inclined to act selfishly with regard to how much sustenance it ought to receive from its mother. In demonstrating this tendency, David Haig, a biologist working at Harvard University, has documented how fetal cells invade the main artery supplying blood to the placenta, killing the artery’s muscle cells. The goal of these killer fetal cells is to usurp control of the blood supply from the mother. The fetus also secretes human placental lactogen (hPL), a hormone that blocks the effects of insulin and thereby increases the amount of glucose in the mother’s blood.
The fetus is actually pursuing a precarious strategy, attempting to reduce the mother’s energy stores at the expense of future siblings, but not to the extent that the mother cannot successfully breast-feed following the birth. Because the mother is equally related to all of her offspring, she prefers to retain more of her energy reserves for future offspring and resists the fetus’s selfish efforts by secreting ever higher levels of insulin. Moreover, the gene that codes for the fetus’s production of hPL turns out to be inherited from the father, not the mother, whose biological interests are ill served by this inimical substance. Indeed, the fetus can be considered a “paternal parasite” growing inside the mother.
From an evolutionary perspective, pregnancy is thus the story of steady escalations in selfish fetal actions followed by equally selfish maternal reactions. Usually this arms race is evenly balanced (owing to the Red Queen effect), and pregnancy runs its normal course. But sometimes there are complications. When expectant mothers suffer from high blood pressure and gestation diabetes, these are often medical instances in which the tug-of-war has become dangerously imbalanced in favor of the fetus. Thus maternal investment in the fetus—itself a special case of altruism toward close relatives—is plagued by continual conflicts based on the differing genetic interests of mother and offspring. These and other fascinating biological findings are reviewed by Ridley as he documents how genetic cooperation risks occasional rebellion on the part of inherently selfish genes.
It is easy for Darwinians to explain cooperation between cells, because cells almost always are genetically identical to one another within the same organism. Similarly, cooperation between close genetic relatives is explained by Hamilton’s theory of inclusive fitness. But why does cooperation occur among unrelated organisms? One of Darwin’s seminal ideas has again provided modern biologists with the critical stimulus. In the early 1970s Robert Trivers, a brilliant young biologist at Harvard, revamped Darwin’s speculations about reciprocal altruism using a formal cost-benefit approach to the problem.
Ridley illustrates this theory with a vivid biological example first identified by Trivers. “Cleaner-fish” live in coral reefs, where they boldly enter the mouths of larger “client-fish” to remove parasites. Remarkably, client-fish do not eat their cleaners. This behavioral relationship is dependent on an inherited disposition, because client-fish raised in isolation (and accustomed to gobbling up smaller fish for dinner) will instinctively open their mouths for cleaning the very first time they encounter a cleaner-fish. Cases like that of the cleaner-fish demonstrate that cooperation can evolve among unrelated organisms as long as there is mutual benefit. Used in conjunction with Hamilton’s theory of kin selection, the idea of reciprocal altruism has greatly increased the scope of Darwinian approaches to cooperative behavior and is especially important for understanding primate social behavior.
In the early 1970s, work on reciprocal altruism was combined with the study of game theory, introduced from the field of economics. Much of Ridley’s story is built around this fruitful episode of intellectual cross-fertilization. At the heart of his account is the well-known game called the prisoner’s dilemma. Two prisoners—both implicated in the same crime—are given a chance to lighten their sentences by testifying against each other. They are kept in separate cells and cannot communicate. Without their testimony, the authorities can convict them only on a lesser charge, punishable by two years in prison. By giving evidence, an informant can avoid doing jail time, but his partner’s sentence will be increased to ten years. If both prisoners remain silent, they are collectively better off than if both defect; but each is individually better off if he defects, no matter what the other one does. The “rational” choice, then, is always to defect—or so asserted the economists through the 1970s.
As game theorists became increasingly intrigued by the prisoner’s dilemma during the 1970s, they set about transforming the predicament to make it more lifelike. For example, theorists altered the rules so that people could play the game more than once. Players were also allowed to communicate with one another during the game. Under such conditions, researchers found that experimental subjects were much more willing to cooperate, a result that some economists initially dismissed as evidence of “irrational” behavior, given that the “rational” choice in the prisoner’s dilemma is always to defect. In 1979, the political scientist Robert Axelrod sponsored a computer tournament for games similar to the prisoner’s dilemma. To everyone’s surprise, the tournament was won by the simplest of all the programs submitted. Called “Tit-for-tat,” the victorious program always began by cooperating with its competitor programs and then faithfully repeated whatever the other programs had last done.
One of Ridley’s most dramatic examples from natural behavior helps to illustrate the virtues of this strategy. In Central America, vampire bats feed at night on the blood of large animals. Not every bat is successful in finding a meal. Without blood, a bat can starve to death in about three days. Vampire bats have solved this problem by resorting to conditional cooperation. Back in their dens, bats who have found a meal during the night regurgitate blood into the mouths of bats who have not. Recipients of these nightly favors tend to return them on subsequent nights to those bats who have fed them in the past. Conversely, bats often refuse another bat who has denied them in the past. The key to this bit-for-bat process is the individual bat’s ability to remember the history of its relationships with all other bats living in its den. This mnemonic requirement has driven the evolution of vampire bat brains, which possess the largest neocortex of all known bat species.
The story of vampire bats underscores an important part of Ridley’s argument. Evidence for reciprocal altruism is relatively rare in animals, with the exception of dolphins, porpoises, and our closest primate relatives. Memory for past actions—and hence the ability to discriminate between past altruists and defectors—requires a large brain. Among social animals, there is a direct correlation between the size of the neocortex and the size of the typical group. Human beings are consistent with this general trend, possessing the largest neocortex and also living in the largest social groups of any primate.
Nature versus Nurture
There is ample evidence that humans cooperate with people to whom they are not closely related—more so than for any other species. Following his relentless Darwinian logic, Ridley attributes these cooperative tendencies to an innate propensity toward “virtue.” This claim would obviously be false were it about unconditional virtue. Humans, however, have evolved dispositions to cooperate or compete that take their cues from the actions of other individuals. In positing an innate but contingent tendency toward virtue, Ridley is fully aware that many aspects of human behavior can be explained just as well by social learning as by genetic predisposition. He also notes that the two explanations are not mutually exclusive. “For human beings,” he acknowledges, “you can never entirely reject the culture hypothesis.” Ridley further cites Daniel Dennett’s argument that ingenious adaptations, if they are really that good, “will be routinely rediscovered by every culture, without need of either genetic descent or cultural transmission of the particulars.”7 Sociobiologists, for example, have sometimes claimed that humans possess a territorial instinct. But the need for defending a territory against rival groups makes so much sense that humans may have repeatedly adopted the practice based on cultural traditions passed down through the generations.
Daniel C. Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life (Simon and Schuster, 1995), p. 487. The capacity for learning, which is so pronounced in our species, is itself a product of natural selection, just as is our capacity for culture. For this reason, the fact that a specific adaptation is achieved by learning is not a decisive argument against there also being a genetic disposition toward such learning. Language acquisition provides a case in point. See Steven Pinker, How the Mind Works (Norton, 1997).↩
Daniel C. Dennett, Darwin’s Dangerous Idea: Evolution and the Meanings of Life (Simon and Schuster, 1995), p. 487. The capacity for learning, which is so pronounced in our species, is itself a product of natural selection, just as is our capacity for culture. For this reason, the fact that a specific adaptation is achieved by learning is not a decisive argument against there also being a genetic disposition toward such learning. Language acquisition provides a case in point. See Steven Pinker, How the Mind Works (Norton, 1997).↩