The key to a con is not that you trust the conman, but that he shows he trusts you. Conmen ply their trade by appearing fragile or needing help, by seeming vulnerable.First you want to help the con artist -- often in return for thoughtful gesture on his or her part. According to the neuroscience, greed kicks in only after you feel the desire to "reciprocate the trust" shown in you by the con artist.
How to avoid con-artists? It's easy to avoid the con artist once you how their games work. Know the typical games in the countries where you plan to visit: deals on jewelery in Bangkok, real estate in Bali, etc. In any country, be wary of invitations to step inside bars and nightclubs.
Another approach is to turn own emotions into a kind of radar system. That is, when you feel emotions such as an overwhelming desire to reciprocate trust or the desire to seize a "rare and fleeting opportunity" in the presence of a stranger, you know its time to make a break for it. You just have to get away, even though it is likely to make you feel like a jerk (at the time).
My laboratory studies of college students have shown that two percent of them are "unconditional nonreciprocators." That's a mouthful! This means that when they are trusted they don't return money to person who trusted them. . . What do we really call these people in my lab? Bastards. Yup, not folks that you would want to have a cup of coffee with. These people are deceptive, don't stay in relationships long, and enjoy taking advantage of others. Psychologically, they resemble sociopaths. Bastards are dangerous because they have learned how to simulate trustworthiness. . .Based what I have seen traveling, it is evident to me that the so-called "bastards" Zak describes are not so evenly distributed by geography as in Zak's college classroom. There can be little doubt bastard is over-represented in urban -- and especially -- touristic places.* In places crawling with bastards, neither of my above-stated rules is sufficient to save you from exasperation.
But, don't be too vigilant: two percent of bastards isn't so bad.
In such places, because anyone who approaches you is likely to be out to manipulate you (perhaps not criminally, but nonetheless cunningly), I only deal with locals who have not approached me first. I find that only the strictest application of this rule makes visiting a city like Phnom Penh enjoyable.
* The converse also seems to be true. Get off the beat-and-path, visit some rural areas of the developing world -- those places that have seldom seen a tourist -- and you are unlikely to encounter anything but kindness and generosity from local residents. Hat-tip: Sullivan and Schneier