Research by evolutionary biologists indicate that both men and women resist their biological programming when they try to be monogamous — something that must be learned, not something that comes natural.
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Why People Cheat
...when people win against others, they tend to think they're better, or more deserving. And that thinking helps them justify cheating, since, after all, they're the rightful heir to whatever throne is next.
While no one knows precisely what went through this mind or motivated him, there was at least one thing that could have hinted at what drove Armstrong to cheat: He was a winner.
The nature of a person’s employment is also related to infidelity—individuals whose work involves touching other people, having personal discussions, or a great deal of one-on-one time are more likely to have an affair.
Men are more likely than women to commit infidelity, largely because men have more testosterone, which is responsible for the strong desire to have sex.