Altruism puzzled Charles Darwin. If evolution was propelled by natural selection – by the gradual predominance within species, including Homo sapiens, of those individuals best adapted genetically for survival and reproduction – where do generosity and compassion come from? Why don’t societies grow more pitiless and cutthroat over time? How did selflessness, charity, and integrity ever establish a foothold, let alone become moral virtues that all enlightened communities esteem?

Darwin could see the clear evolutionary benefit to groups that inculcated ethical values in their members. Imagine, he wrote in The Descent of Man, two competing primitive tribes, equally matched – except that “one tribe included a great number of courageous, sympathetic, and faithful members, who were always ready to warn each other of danger, [and] to aid and defend each other.” There was little doubt that tribes highly endowed with such virtues “would spread and be victorious over other tribes.”

But there was a problem: How did any tribe evolve such ethical qualities in the first place? Brave individuals who risked their lives for others “would on average perish in larger numbers than other men.” It hardly seemed possible, Darwin conceded, that “such virtues … could be increased through natural selection, that is, by the survival of the fittest.”

Darwin’s paradox has generated a vast literature in evolutionary psychology and sociobiology. Scientists have demonstrated that humans have a hard-wired moral capacity; we are born with an aptitude for empathy and fairness that is built into our biology. Recent neurological experiments, for example, demonstrate that an act of generosity, such as donating to charity, triggers a pleasurable response in the brain.

Of course, having a capacity is not the same as using it. The human brain is hard-wired to learn multiple languages, too, but how many of us ever master more than one? Our moral sense may be genetically encoded, but we aren’t robots. We have free will. Each of us must choose to be decent or indecent. And there is no denying that indecent choices can also convey rewards.

Thus did Darwin put his finger on what Sir Jonathan Sacks calls “the central drama of civilization: Biological evolution favors individuals, but cultural evolution favors groups…. Selfishness benefits individuals, but it is disastrous to groups, and it is only as members of a group that individuals can survive at all.”

Sacks, who retired this week after 22 years as Britain’s Orthodox chief rabbi, writes in his recent book The Great Partnership – an eloquent argument about the interdependence of religion and science – that Abrahamic monotheism has provided the most enduring solution to Darwin’s conundrum. “Religion … creates altruism, the only force strong enough to defeat egoism.”

As Jews this week celebrate Rosh Hashana, that message will be renewed.

In the Jewish tradition, Rosh Hashana marks more than the start of a new year. It is the day when God sits in judgment over mankind, weighing each person’s deeds and misdeeds, deciding who will live and who will die, whose year will be tranquil and whose tormented. More than at any other time in Jewish life, the 10 days from Rosh Hashana through Yom Kippur, the Day of Atonement, reinforce the conviction that everything we do is seen and recorded, and that we are called to account for our choices.

Yet God is not immovable. Over and over, the High Holiday liturgy stresses the power of repentance to overturn a harsh decree. And repentance is achieved not just through prayer but through acts of charity, goodness, and honesty – through behavior changed for the better.

You don’t have to be Jewish, or even a believer, to be grateful for such moral reinforcement. “I want my attorney, my tailor, my servants, even my wife, to believe in God,” declared Voltaire, a notorious religious skeptic, “for I think I shall then be robbed and cuckolded less often.”

However cynical, Voltaire was right. Human beings are more likely to do the right thing if they think they’re being watched, which is why speeding drivers slow when they see the flashing lights of a police cruiser, and why visible security cameras reduce shoplifting. Surveillance changes behavior. Even the illusion of surveillance – a cardboard police officer, a poster of staring eyes – can make people more honest. Religious believers know that they are being watched not by an inanimate camera or a poster, but by an all-knowing God who calls us to love our neighbor, feed the hungry, and pursue justice.

“God sees. Therefore we are seen,” writes Sacks. That is a powerful spur to improve our deeds; to take right and wrong seriously; to be honest and ethical even when others aren’t. Our genes may be selfish. We can aspire to something higher.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).