There is this thing called Fundamental attribution error in psychology.
http://en.wikipedia.org/wiki/Fundamenta ... tion_error
Pretty much everyone who believes "once a cheat always a cheat" is suffering from it.
People make actions with the major influencing factor being circumstances. If the circumstances needed to make someone cheat, (bordedom, lack of sexual ful fillment, lack of commitment, fear of entrapment) are not present. Chances are this person will not cheat.
However if those (or some) of those circumstances are in place, then a person will probably cheat.
Problem is when people are cheated on they rarely want to accept that something with the relaitonship was wrong. And instead become motivated to commit Fundamental attribution error, by claiming that the problem was something in-herint in their partner.
Thus relieving them of any real responsibility, painting themselves as someone who was doing everything they could and was just "tricked".
98% of the time, a bit of close inspection will reveal that that was not the case.
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