Psychology Today, as discussed by Bruce, provides insight into the mind of cons and the practice of fraud:

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 (these experiments are described in my post on neuroeconomics). 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. My research has demonstrated that they have highly dysregulated THOMASes [The Human Oxytocin Mediated Attachment System].

The author emphasizes that two percent is not bad since that means a large majority of people therefore are not bastards. He also turns to literature for historic prose on living with fraud:

Russian playwright Anton Chekov said “You must trust and believe in people or life becomes impossible.” I’d say that’s about right-just watch for the occasional con.

Occasional is higher than two percent, seems to me. What percentage would Chekov have guessed? Does the percentage go up in later age groups?

A 2002 paper called Trust among strangers is based on a game that simulates the opportunity for reciprocation and charts the probabilities.