When New York Life offers—with “no obligation”—a flashlight and keychain to AARP members who agree to let the company send them information, it’s employing a scientifically proven persuasion principle.

Same with TruGreen, when it solicits lawn-care business in a mailer that says “57 of your neighbors have TruGreen lawns.” Ditto for Core Power, when it becomes the “official protein drink” of the 2014 Sochi Winter Olympics.

People speak of the art of persuasion, but many companies and individuals use methods—consciously or not—that behavioral science has shown are effective, whether in making a sale, gaining cooperation or consensus, or coaxing charitable donations, says Robert Cialdini, professor emeritus of psychology and marketing at Arizona State University. He’s the author of Influence: The Psychology of Persuasion.

“Using these principles well is now a competitive advantage, and more companies are starting to hire people who can do so,” says John Balz, behavioral marketing manager at Opower, a Virginia-based clean technology company.

Cialdini tells of his surprise when he read that impoverished Ethiopia had sent $5,000 for relief aid to Mexico in the aftermath of a devastating earthquake there in 1985. He subsequently learned that 50 years earlier, Mexico provided aid to Ethiopia when it was invaded by Italy.

Reciprocation occurs on a smaller scale constantly. Vendors who provide free samples of their products hope people accepting them will feel compelled to reciprocate by making a purchase. Same for nonprofits that mail personalized address labels and calendars when soliciting contributions.

One study found that diners tipped a higher percentage when their server left each of them a piece of candy with the bill than when they received only the check. When the server left two pieces of candy, the diners left an even larger tip. And when the server added a personal touch by offering each diner one candy, then stopping and offering them one more piece when leaving their table, the tip was highest of all.

“The explanation that appears to be the most plausible in explaining the candy effect is the norm of reciprocity,” the researchers concluded.

2. Commitment and Consistency

Nearly a half-century ago, two psychologists asked residents of a California neighborhood for permission to erect a huge billboard in their front yards urging motorists to drive safely. Most said no, of course. But about three-quarters of one subset of the residents agreed to the invasive request. They had previously agreed to display a 3-inch-square sign related to driving safely and, by saying yes to the billboard, they were honoring their commitment to this cause, Cialdini says.

We try to be consistent with what we’ve previously said or done, especially if it’s in writing. Research shows that folks who sign a petition are more apt to work on behalf of the position because of their written commitment to it. “People live up to what they write down,” Cialdini says.

Some advertisers purposely omit key information in a variation of the commitment principle. When you read or hear an ad for a concert that doesn’t disclose ticket prices, for example, you are more likely to buy tickets if you go to the trouble to find out the costs. The reason, Cialdini says, is that you have already made an initial commitment to attending by taking the extra step to learn  more.

The commitment and consistency principle presents a challenge when wooing a prospective customer or client who has been using a rival company for a long time. The prospect may stubbornly stick with the status quo out of loyalty and to justify his or her initial commitment. “It’s not ever wise to say to them, ‘You’re making a big mistake by going with this provider. We have a better product,’ ” Cialdini says. “That causes defensiveness.” A better approach: “Circumstances have changed. We now have a better product.”

3. Social Proof

Hotel rooms often include messages about the environmental benefits of guests reusing towels during their stays—and not incidentally, reducing the hotels’ laundry and energy bills.

The messages are effective, but Cialdini and others took it further and did a study in which hotel guests were informed that a majority of fellow lodgers had reused their towels at some point during their stays, rather than the simple environmental message. These guests were 26 percent more likely to reuse their towels. And when they were told that a majority of prior guests in their particular room had reused towels, compliance rose 33 percent.

“People want to follow the lead of what those around them are doing,” Cialdini says. That’s social proof.

It has worked at Opower. The company sells software to utility companies that enables them to inform customers how their energy use compares with similarly sized homes in their areas as well as the most energy-efficient homes. “The idea is to [encourage] people to reduce their energy use,” Balz says.

Those who are doing better at conserving than their neighbors get one additional feature on their reports: a smiley face. “It’s very important for people who are doing well to be recognized for it and encouraged to continue to do it. It’s a key piece for the most motivated customers” to continue their efficient ways, Balz says.

4. Authority

Ad agencies know we tend to follow the advice of knowledgeable, credible people. Who hasn’t heard or read claims that three out of four doctors recommend a drug, or nine out of 10 experts agree that exercise is good? (Never mind that in 1946, the RJ Reynolds tobacco company initiated an ad campaign with the slogan, “More doctors smoke Camels than any other  cigarette.”)

Movie ads that feature positive blurbs from critics employ the authority principle. Even people who display symbols of authority—an extravagant car, a grandiose title or attire such as a uniform or an expensive business suit—often are treated with more deference.

One study, for example, found that motorists were not as quick to honk when they were stuck behind drivers of luxury cars, rather than those in economy vehicles, which were stopped at green  lights.

Then there’s faux authority in which individuals with a veneer of expertise peddle a product. Alex Trebek capitalizes on his erudite image as host of a game show for smart people—Jeopardy!—to pitch life insurance in Colonial Penn commercials.

Cialdini cites actor Robert Young’s old TV commercials for Sanka decaffeinated coffee. The star of Marcus Welby, M.D. from 1969 to 1976 didn’t wear a white lab coat for the ads, but he extolled the health benefits of eliminating caffeine from one’s cup of java. Sanka replaced Young after six years with commercials featuring people from various walks of life singing Sanka’s praises. With that, it switched from the authority principle to that of social proof.

Internet sites like Amazon and Yelp are changing the nature of these authoritative testimonials, Cialdini says. Now consumers can easily read reviews and evaluations from people like themselves.

5. Liking

It seems obvious that if your company or you are likable, your odds of exerting influence improve. Yet it’s not that simple, Cialdini says. “We like those who like us. [But] how we demonstrate commonalities isn’t so widely known. That requires a research base.”

For example, people have more influence when they match the verbal style and idioms of those with whom they are talking. A study showed that waitresses who repeated customers’ orders back to them verbatim saw their tips double, Cialdini says.

We also like people who are similar to us. Researchers at Santa Clara University did a series of experiments with undergraduate students that demonstrated this. In one, participants who thought they shared a birthday with a second person agreed much more often to the latter’s request that they read and critique her lengthy essay than those with no birthday connection.

The more unusual the similarity, the stronger we identify with others. In a second study, participants who thought they had the same rare type of fingerprints as a second person agreed even more than those in the birthday experiment to read and critique the latter’s essay.

Sincerely complimenting others also spurs liking, as does cooperating with them toward a common goal, Cialdini says.

Linking yourself or your product to something positive also makes you or it seem more likable. Since the 1930s, General Mills has featured athletes on the front of Wheaties boxes, bolstering its likability and its “Breakfast of Champions” tagline. The athletes selected must be “champions on and off the field,” says David Oehler, Wheaties marketing manager. “It is something consumers continue to ask for. We know there’s still an appetite for that.”

6. Scarcity

“As strange as it seems, the best way to get people to try, sample or buy is to somehow limit their ability to do so,” Joshua Weltman writes in his just-published book, Seducing Strangers: How to Get People to Buy What You’re Selling. He’s a veteran creative director who is advertising consultant and co-producer of the drama series Mad Men.

Car dealer ads that claim only one particular vehicle remains on the lot at a very low price exemplify the scarcity principle.

“Urgency is also created when people feel they need to act before an opportunity gets away,” Weltman writes. One method is limited-time offers, which he says are aimed at market shoppers who are interested in buying and are just waiting for the right opportunity.

While limited-time offers are persuasive, companies may “get hooked on the bump and want to run them all the time,” Weltman writes. When that happens, “the sensation of urgency dulls in the public’s perception.”

Studies show we value items that have recently become restricted more highly than those that have been scarce all along, Cialdini says. And we’re more attracted to limited items when we compete against others for them. Think Black Friday.

Cialdini points to a study in which participants who tasted a chocolate-chip cookie from a jar that contained two of the treats rated its quality more highly than those who sampled the same cookie from a jar containing 10 of them.

Some of the participants in the two-cookie group were told that a portion of their cookies had been given to other evaluators to supply demand for the study, while others were informed their cookies had been reduced because of a researcher’s error. The raters who believed other tasters received some of the cookies ranked them significantly higher than their counterparts.

Regardless of which persuasion principles one relies on, Cialdini emphasizes that they should be used ethically. To do otherwise, he says, “makes bad business sense.”

Lying is especially foolish now that criticism can travel fast and far via social media, Weltman says. “Make a good promise for a bad product to people connected by social media and watch business implode at the speed of light,” he  warns.

Cialdini cautions that the biggest mistake one can make is to assume that one principle is useful in every situation. “That’s a fool’s game to think the same tactic will work in every setting, every audience,” he says. “But in the realm of human behavior, there can be no certainties.”