As many as 90 percent of Americans believe infidelity is unacceptable, yet 30 to 40 percent of people engage in it. When talking about the reasons for such toxic relationships, Kelly Campbell, Ph.D., a Professor of Psychology at California State University, categorizes them into 3 groups. Individual reasons, relationship reasons, and situational reasons.

"The phrase 'once a cheater, always a cheater' refers to individual reasons for cheating—qualities about the person that make him or her more prone to commit infidelity," the professor wrote for Psychology Today. "Researchers have identified a variety of individual risk factors, including gender, personality, [as well as] religiosity and political orientation."

Interestingly, men are more likely than women to commit infidelity, primarily because they have more testosterone, which is responsible for fueling the desire to have sex.

People who have less conscientious and less agreeable personalities are also more likely to cheat.

On the other hand, deeply religious people and those with a conservative political orientation are less likely than others to commit infidelity because they have more rigid values.

"People also cheat because of relationship reasons—characteristics about their relationship itself that are unsatisfying," Campbell said. "For these people, becoming involved in a well-matched partnership diminishes or eliminates their desire to cheat. 'Once a cheater, always a cheater' does not hold true for this group. When they stray, factors about the relationship itself must be examined. Researchers find that partnerships characterized by dissatisfaction, unfulfilling sex, and high conflict are at higher risk for infidelity. Also, the more dissimilar partners are—in terms of personality, education level, and other factors—the more likely they are to experience infidelity."

Others cheat because of the situation they're in. "A person might not have a personality prone to cheating and might be in a perfectly happy relationship, but something about their environment puts them at risk of infidelity. Some situations are more tempting than others. Spending time in settings with many attractive people can make cheating more likely. 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."

"When the sex ratio is imbalanced (an overabundance of men or women in the work or campus environment), people are also more likely to experience infidelity. Finally, people who live in urban areas, as opposed to rural, less populated regions, are at greater risk—people in metropolitan locations generally have more liberal attitudes about extramarital sex, and cities simply have more people, creating an environment of higher anonymity and a larger potential group of partners with whom to have sex."

If you want to protect your relationship from cheating, Campbell advises talking to your partner about the definition of infidelity. "People have different ideas about what constitutes cheating and partners need to develop consensus." It is easier to understand where the boundaries are when everyone's on the same page. While most couples can agree that sex definitely counts as cheating, it might be beneficial to discuss more nuanced behaviors.