Robert Cialdini knew himself to be a sucker for almost every sales ploy that came along, from the Boy Scout selling candy bars to the door-to-door pitchman pushing encyclopedias.

Being a psychology professor at Arizona State University, he knew there must be a reason he and other people say "yes" to friends and even strangers who have items for sale.After all, he doesn't even like chocolate, and he didn't need another set of expensive reference books.

This led him to study people whose business it is to make others say "yes."

"I actually became an undercover agent, a spy of sorts, where I infiltrated as many naturally occurring influence professions as I could get access to," Cialdini, 42, said recently. "I took training in door-to-door sales. I trained to sell portrait photography over the phone, automobiles and other kinds of goods."

He answered newspaper ads for sales trainees and went through the training programs to learn the tricks of the trade.

He interviewed labor-relations negotiators and recruiters for such religions as the Unification Church and the Hare Krishna Society.

Finally, Cialdini talked to police bunco officers to learn how con artists talk seemingly intelligent people into harebrained and expensive schemes.

What I tried to do when I had all this information was to look for parallels, for factors that were in common, certain psychological principles that each one of these professions used effectively," Cialdini said. "That told me these were the principles that were central to motivating people toward 'yes.'"

He found six reasons people say "yes" to the requests of other people. Each reason became a chapter in his book, "Influence: The New Psychology of Modern Persuassion," published in 1984.

"Take reciprocation, the first chapter in my book," he said. "People feel obligated to give back if they receive something, so that a requester can greatly increase the chance he or she will get a 'yes' to his or her request by first doing something or giving something."

The Hare Krishnas give flowers and the Disabled American Veterans send unsolicited return-address labels before asking for a donation. Then there are the women in supermarkets who look like someone's auntie and give away samples of processed meats or cheeses on toothpicks.

"I can't give her back just the toothpick," Cialdini said, holding up an imaginary toothpick between his thumb and forefinger. So he usually ends up buying some of the meat or cheese being promoted.

He lists five other principles of influence.

--Commitment and consistency: Once a commitment is made, there is a natural tendency to behave in ways that are subbornly consistent with that commitment.

A customer agrees to purchase a car because of a "lowball" price. He or she remains committed to the deal even after the sales manager finds an "error" in the salesman's calculations and other add-on costs are found.

__Social proof: When a person sees others doing something, he assumes it must be the right thing to do. A person is even more likely to follow the lead of someone like himself.

--Liking: People like too say "yes" to people they know and like.

__Authority; We are trained from birth that obedience to proper authority is right. Symbols such as clothing, titles and expensive cars may be used to imply authority.

--Scarcity: Anything that becomes less available seems more desirable, especially if we must compete for it.

Although these principles can be exploited, Cialdini says these rules of behavior are essential tools for making decisions.

"They're good for society," he said. "Each of these (principles) works well for ourselves and society most of the time because they are such good shortcuts."

When people use these six shortcutss to make decisions, they are reacting to the stimuli presented and are not thinking very much, Cialdini said.

The problem occurs when others, knowing we are simply reacting, exploit the system, he said.

"Once you know what those (reasons) are, and whenever you see someone using one of those techniques on you, you have to stop and take a step back from the situation and say, "What are the pros and cons of this situation? I have to stop simply reacting and think about the merits of what I'm being offered,'" Cialdini said.

"In other words, separate the thing from the presentation of the thing..."