In an advertising campaign that began last week, Nissan left 20,000 sets of keys in bars, stadiums, concert halls and other public venues. Each key ring has a tag that says: “If found, please do not return. My next generation Nissan Altima has Intelligent Key with push-button ignition, and I no longer need these.”

This campaign is clever, but not particularly original.

It was 1997, and the man who was crouched on the sidewalk at 68th and Broadway in New York City was one of the most pathetic souls I’d ever seen. His limbs were twisted in what appeared to be arthritic agony and tears were streaming down his face. “Please,” he whimpered. “Please, somebody help me.”

Most passers-by did what they were named for, but my wife and I stopped. The man looked up. “Please,” he sobbed. “I just want to go home.” My hand needed no guidance from my brain as it reached into my wallet and extracted $10. “Thank you,” he said as I handed him the money. “Thank you so much.” My wife and I mumbled some embarrassed words and walked on.

We hadn’t gone a block when she tugged my sleeve. “Maybe we should have gotten him into a cab,” she said. “He could barely stand up. He might need help. We should go back to see.” My wife is the patron saint of lost kittens and there is no arguing, so we went back to see. And what we saw was our horribly crippled friend walking briskly and happily up 68th Street, opening the door to a late-model car, getting in and driving away after what was apparently a short day of theatrical work.

I know two things now that I didn’t know then.

First, I now know that my hand did what human hands were designed to do. Research suggests that we are hard-wired with a strong and intuitive moral impulse —- an urge to help others that is every bit as basic as the selfish urges that get all the press. Infants as young as 18 months will spontaneously comfort those who appear distressed and help those who are having difficulty retrieving or balancing objects. Chimpanzees will do the same, though not so reliably, which has led scientists to speculate about the precise point in our evolutionary history at which we became the “hypercooperative” species that out-nices the rest.

The second thing I know now that I didn’t know then is that this was the most damaging crime I had ever experienced. Like most residents of large cities, I’d been a victim before —- of burglary once, of vandalism several times. But this was different. The burglars and vandals had taken advantage of my forgetfulness (“Why didn’t I double lock the door?”) and taught me to be better.

But the actor on 68th Street had taken advantage of my helpfulness and taught me to be worse. The hand that had automatically reached for my wallet had been slapped, and once slapped was twice shy. I’ve never again given money to a stranger without scrutinizing him for the signs that distinguish suffering from its imitation. And because I don’t know what those signs are, I typically just walk by.

Now corporate America has taken a lesson from the guild of shameless grifters. Nissan’s plan to leave those 20,000 sets of keys in public venues is every bit as crafty as the fraudulent performance that a decade ago left me with holes in both my pocketbook and soul. There is no selfish reason to bend down and pick up a key ring, but Nissan knows that we will bend without thinking because the impulse to help is bred into our marrow. Our best instinct will be awakened by a key ring and then punished by a commercial. Like rubes throughout the ages, we will be lured by a false cry of distress and quickly cured of our innocence and compassion.

We are used to commercial tricks that play on our fears. The official-looking letter marked “Verification Audit” is actually a magazine subscription renewal form; the credit card company’s ominous call to “discuss your account” is actually an attempt to sell new services.

Should we now get used to commercial tricks that play on our humanity? How would we feel about a device planted in trash bins that screams “I’m stuck!” until the lid is opened, at which point it continues, “Stuck in a dead end job, that is —- and if you are too, then let us show you how to make millions in real estate with no money down”? Is it O.K. to send a thousand doleful puppies into the streets with tags that say: “Thanks for checking. And speaking of checking, our bank charges no monthly fees”?

What happens to us when greed masquerades as need, when cries for help become casting calls for chumps, when our most noble actions make us patsies? “You put an idea out there and seed it,” said the president of the advertising agency that came up with Nissan’s key ring ploy. “And people carry it for you.” Indeed they do. The idea being seeded and carried in this case is that the world cries wolf, that our moral impulse betrays us and that smart people should keep on walking.

You can learn more about the “the moral impulse” by reading

• F. Warneken and M. Tomasello (2006). Altruistic helping in human infants and young chimpanzees. Science, 311, 1301-1303.

• Haidt, J. (2001). The emotional dog and its rational tail: A social intuitionist approach to moral judgment. Psychological Review, 108, 814-834.