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The biological roots of sin are not by themselves a news flash from the frontiers of science. More than a century ago, Thomas Huxley, Darwin's popularizer, lamented the fact that evolution has given all children "the instinct of unlimited self-assertion"--"their dose of original sin." But the past few decades have brought a deeper Darwinian understanding of human nature, and some of its pioneers believe Huxley underestimated our badness.
But first the good news. Evolutionary psychologists say our "moral sentiments" do, as Darwin speculated, have an innate basis. Such impulses as compassion, empathy, generosity, gratitude and remorse are genetically based. Strange as it may sound, these impulses, with their checks on raw selfishness, helped our ancestors survive and pass their genes to future generations.
Now the bad news. Contrary to a misconception that flourished in Darwin's day, these impulses did not give this boost to genetic proliferation mainly by furthering the overall "welfare of society"--and certainly not by furthering the "welfare of the species." As a result, humans don't naturally deploy our "moral" impulses diffusely--showering love and compassion on any needy Homo sapiens in the vicinity. We tend to reserve major doses of kindness either for close kin (the result of an evolutionary dynamic known as "kin selection") or for non-kin who show signs of someday returning the favor (a result of the evolution of "reciprocal altruism"). This finickiness gives our "moral" sentiments a naturally seamy underside. Beneath familial love, for example, is malice toward our relatives' rivals. Remember the woman in Texas who plotted to kill the mother of her daughter's rival for a cheerleading slot? Fortunately, she's an extreme example. But she's an example, nonetheless--as any father or mother, on honest reflection, will admit.
The grimness of this new Darwinian worldview has been stressed by the biologist George Williams, whose 1966 book Adaptation and Natural Selection laid its theoretical foundations. Rather like the World War II physicists who were horrified by the weapon they had invented, Williams blanches at the view of human nature and of natural selection that he helped usher in. "Mother Nature," he says, "is a wicked old witch."
Perhaps the most disturbing aspect of our nature is our natural blindness to it. According to some evolutionary psychologists, we are "designed" by natural selection to conceal selfish motives from ourselves--indeed, to unconsciously build elaborate moral rationales for our selfish behavior. Thus do wars routinely feature two sides convinced that they are in the right.
Even at the level of pettier sins--lust, greed--we are naturally good at making our actions seem just. How many spouses are lured into infidelity, even desertion, by the conviction that they married the "wrong" person the first time around or that this person has "changed"? These often delusional rationales are (to put it a bit metaphorically) our genes "trying" to get us to do the kinds of things--infidelity, betrayal--that during evolution helped propel them into the next generation. Hence the human dimension of our animal behavior: it feels so rational and right. Lots of animals are violent, treacherous and nasty, but only one convinces itself that God approves.