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Back when E. O. Wilson first promoted his newly hatched theory of sociobiology, protesters doused him with a pitcher of water. Since then, sociobiology has come a long way, baby. Dolled up with fancy new monikers like “evolutionary psychology,” it now saunters boldly into the academy claiming to be not only a valid field of investigation but much more—a comprehensive synthesis of biology and philosophy, a guide to ethics and policy. Many scholars are now scrambling for the lady’s hand, eager to claim her for their own philosophical programs.

Yet evolutionary psychology turns out to be disturbingly fickle, capable of supporting a wide range of ethical conclusions. For even if we grant that certain human behaviors have an evolutionary origin, that does not tell us which behaviors are normative or morally good. In his latest book, A Darwinian Left, Peter Singer joins the line of suitors, hoping to win over evolutionary psychology to his particular liberal agenda. Whether he succeeds is another matter.

Sociobiology originally raised hackles, Singer explains in an interview, because it was regarded as a revival of “nasty, right wing biological determinism”—a revival, that is, of Social Darwinism, which has long harnessed the idea of the survival of the fittest to notions of progress through competition and the ruthless pursuit of self interest. In evolutionary psychology, the selfish individual has merely been replaced by the selfish gene.

Interestingly, Singer does not deny this unpleasant view of human nature; indeed, he urges the left to face up to its truth. Leftist utopianism is based on the assumption of the malleability of human nature, leading to dreams of “the Perfectibility of Man.” Nor are these merely idle dreams—they have inspired attempts to remake society and human nature, issuing in the totalitarian state.

But Darwinism implies that human nature is not completely malleable, Singer argues: the left must “face the fact that we are evolved animals, and that we bear the evidence of our inheritance, not only in our anatomy and our DNA, but in our behavior too.” Thus humans possess “evolved dispositions”—for example, to act from self interest and to form hierarchical social arrangements—and political thinkers need to take these dispositions into account. Not that Singer thinks leftists should give up their ideal of, say, an egalitarian society; but they should understand that it “is not going to be nearly as easy as revolutionaries usually imagine.”

Yet this dark view of human nature is only half the picture, Singer insists. Recent Darwinians have shown that humans are hard wired by natural selection for cooperative as well as competitive behavior, even for altruism. Singer cites now-familiar studies of kin altruism, where apparently sacrificial behavior on the part of a mother for her child is “explained” as a strategy for passing on her genes. He also describes game theory experiments showing that cooperative strategies—tit for tat—work best in getting what we want. Of course, neither of these examples represents altruism in the ordinary sense; they are merely extended forms of self-interest. Nevertheless, they are enough to satisfy Singer that Darwinism may now be harnessed to support the left’s vision of a more cooperative society.

Does Singer ultimately succeed in crafting a Darwinian left? Not exactly. To begin with, for all his eagerness to be identified as a man of the left, Singer shows a cavalier disregard for the concerns of real leftists. Historically, the left focused on the ownership of the means of production; in today’s “identity politics,” the enemy is no longer capitalism but racism, sexism, and homophobia. Yet Singer says nothing about any of these; instead he offers a definition of the left so broad as to be meaningless. “[T]he core of the left is a set of values,” he writes. A person of the left sees “the vast quantity of pain and suffering that exists in the universe” and wants “to do something to reduce it.” Under this expansive definition, everyone who favors social amelioration—including, no doubt, everyone reading this review—is a leftist.

Clearly, Singer is just not all that interested in leftist political theory. Neither is he very interested in evolutionary psychology, it turns out. For all his eagerness to woo Darwinism away from the right, he recognizes that evolution cannot provide a basis for the “set of values” he wants to defend. Earlier sociobiologists like Wilson had hoped that evolution would reveal “ethical premises inherent in man’s biological nature,” challenging “the traditional belief that we cannot deduce values from facts.” But those earlier hopes have been chastened, and today most proponents of evolutionary psychology vigorously disavow the naturalistic fallacy of seeking to derive “ought” from “is.”

“Evolution carries no moral loading, it just happens,” Singer writes. “Even an evolved disposition . . . cannot serve as the premise of an argument that tells us, without further ethical input, what we ought to do.” Darwinism tells us merely what barriers exist in human nature to enacting a given political agenda, allowing us to better assess costs and benefits; it does not provide a justification for values. And “since to be of the left is to hold certain values,” Singer writes, “Darwin’s theory has nothing to do with whether one is left or right.”

So why did he write the book, one wonders. It turns out that the most important function Darwinism performs for Singer is to debunk certain pre-Darwinian ideas: to wit, the biblical account of human origins and the ethic that goes along with it—especially the idea that humans are unique and ought to be treated differently from nonhumans. That view has been “thoroughly refuted by evolution,” Singer asserts. By positing an unbroken historical continuum from animals to humans, “Darwinian thinking provided the basis for a revolution in our attitudes to non­ human animals.” Thus a Darwinian left would “work towards a higher moral status for nonhuman animals, and a less anthropocentric view of our dominance over nature.” Here we recognize Singer’s familiar profile as a supporter of animal rights (and of euthanasia and infanticide—for humans at least). And here is also where his real interest lies: in supporting an ethic of “impartial concern” for all sentient beings.

But how to support such an ethic? Having ardently courted evolutionary psychology through most of the book, in the final pages Singer drops it suddenly like an old mistress when true love comes along. And true love for Singer is . . . reason. In some unexplained way, natural selection has made us “reasoning beings,” which enables us to transcend the impulses instilled by natural selection. Through reason we are able to develop genuine altruism, not merely kin altruism or enlightened self-interest. “We do not know,” Singer writes wistfully, “to what extent our capacity to reason can . . . take us beyond the conventional Darwinian constraints on the degree of altruism that a society may be able to foster.”

In other words, Darwinian evolution has produced a capacity—reason—that transcends Darwinian evolution. Singer hopes that the insights of reason may eventually “overcome the pull of other elements in our evolved nature” until we embrace “the idea of an impartial concern for all of our fellow humans, or, better still, for all sentient beings.”

Singer doesn’t account for this novel capacity that frees us to act against our evolved nature—he simply pulls it out of a hat. Quoting arch-Darwinian Richard Dawkins, he holds out the prospect of “deliberately cultivating and nurturing pure, disinterested altruism—something that has no place in nature, something that has never existed before in the whole history of the world.” In other words, reason is presented as a mysterious capacity capable of creating something de novo, something that has never existed before—one might even say ex nihilo. With this godlike power, we can rise above our evolutionary origins. “Although ‘we are built as gene machines,’” he says, quoting Dawkins again, “‘we have the power to turn against our creators.’”

The eloquence of Singer’s language signals that here we have tapped his most ardent beliefs. This is not Singer the “thinking machine,” as he has been labeled for his cool, calculating utilitarianism regarding euthanasia and infanticide. No, this is Singer the true believer. Here reason is treated as far more than a utilitarian instrument. It is nothing less than the means of achieving freedom—metaphysical and moral freedom. Singer alludes to Hegel, who “portrayed the culmination of history as a state of Absolute Knowledge, in which Mind knows itself for what it is, and hence achieves its own freedom.” We don’t have to buy Hegel’s metaphysics to see that “something similar really has happened in the last fifty years,” Singer enthuses: “For the first time since life emerged from the primeval soup, there are beings who understand how they have come to be what they are.” In short, through scientific rationality, Hegel’s vision of absolute freedom now shows promise of realization: “In a more distant future that we can still barely glimpse, [scientific knowledge] may turn out to be the prerequisite for a new kind of freedom.”

This is an astonishing finale to a book that is otherwise sober and restrained. Singer prides himself on being a realist, offering “a sharply deflated vision of the left, its utopian ideas replaced by a coolly realistic view of what can be achieved.” That may be true when he describes the biological constraints on human possibilities. But when he promises “a new kind of freedom” from those same biological constraints, he becomes a flaming utopian, as passionate as any revolutionary.

In the end, Singer’s hope of giving the left a solid basis in science fails—not merely because of ongoing debates over whether Darwinism really explains human behavior but because Darwinism is ultimately irrelevant to his moral vision. Darwinism has its uses in debunking Christian theology, but when it comes to constructing his own ethic, Singer makes a leap of faith to a mystical notion of reason that transcends Darwinian biology.

Needless to say, such a leap renders Singer’s position hopelessly self-contradictory. For the same Darwinian premise that undercuts morality by rendering all behavior merely survival strategies, also undercuts epistemology by rendering the ideas in our minds likewise merely survival strategies. As Richard Rorty has written, “keeping faith with Darwin” means understanding that the human species is not oriented “toward Truth,” but only “toward its own increased prosperity.” Truth claims are just tools to “help us get what we want.” Or as Patricia Churchland puts it, an improvement in an organism’s cognitive faculties will be selected for only if it “enhances the organism’s chances of survival. Truth, whatever that is, definitely takes the hindmost.”

Darwin himself wrestled repeatedly with the skeptical consequences of his theory. Just one example: “With me,” he wrote, “the horrid doubt always arises whether the convictions of man’s mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy.” (Significantly, Darwin always expressed this “horrid doubt” after admitting an insistent “inward conviction” that the universe is not the result of chance after all, but requires an intelligent Mind, a First Cause. In other words, he applied his skepticism selectively: when reason led to a theistic conclusion, he argued that evolution discredits reason. But since reason was also the means by which he constructed his own theory, he was cutting off the branch he was sitting on.)

Similar self-contradictions are endemic in the literature on evolutionary psychology. A prime example is The Moral Animal, where author Robert Wright spends hundreds of pages describing human beings as “robots,” “puppets,” “machines,” and “Swiss watches” programmed by natural selection. He insists that “biochemistry governs all” and that free will is sheer illusion. He unmasks our noblest moral impulses as survival “stratagems of the genes,” as mere devices “switched on and off in keeping with self-interest.” But then, in a grand leap of faith, Wright insists that we are now free to choose our moral ideals, and he urges us to practice “brotherly love” and “boundless empathy.”

This persistent inner contradiction stems from the fact that evolutionary psychology is essentially a search for a secular morality. Darwinism cut the modern world loose from religious traditions and systems of meaning; the result is a culture adrift in a sea of relativism. Now Darwinism is itself being plumbed as a source of meaning, a cosmic guide for the problems of living. Yet the Darwinist view of human nature is so negative, so counter to traditional notions of human dignity, morality, and reason (not to mention common sense), that there is an almost irresistible impulse to take a leap of faith back to those traditional notions, no matter how unsupported by the theory. For who can live with a theory that tells us that “ethics is illusory,” and that “morality is merely an adaptation put in place to further our reproductive ends,” in the words of Michael Ruse and E. O. Wilson? Who can live with a theory that tells us that if “natural selection is both sufficient and true, it is impossible for a genuinely disinterested or ‘altruistic’ behavior pattern to evolve,” in the words of M. T. Ghiselin?

Peter Singer, for one, cannot. One solution would have been to revive the traditional theism that made disinterested altruism a moral ideal in the first place (albeit with a distinction between humans and other “sentient beings”). Instead, he tries to graft that moral ideal onto the Darwinian tree. The graft will not take, and the result is a fatal incoherence.

Nancy Pearcey is a fellow of the Discovery Institute’s Center for the Renewal of Science and Culture and managing editor of the journal Origins and Design . She is coauthor (with Charles Colson) of How Now Shall We Live?