I was struck by a recent study showing that people might be more likely to cheat on a partner in the year before a milestone birthday. This suggests that if you're in a committed relationship, you're at roughly a 10-year cycle for heightened risk of infidelity.

Researchers said they worked with Ashley Madison, a dating website for people seeking extramarital affairs, to analyze data on more than 8 million men who had registered with the site. The study was one of six published together in the journal Proceedings of the National Academy of Sciences in 2014 that examined when people make big life changes. It found 950,000 men were ages 29, 39, 49 or 59, or "9-enders," and their numbers on the dating site were 18 percent higher than what would be expected by chance, according to the researchers from New York University's Stern School of Business and the Anderson School of Management at the University of California, Los Angeles. The study also looked at data for women and found a similar, though less pronounced, pattern.

Infidelity is one of the most complex, least clear-cut areas of relationship research. Most people don't want to admit they have been unfaithful.

Everyone, even the experts, has a different definition of "infidelity." Some define it narrowly as sexual intercourse with someone who isn't your spouse or committed partner. Others define it more broadly to encompass a range of sexual activities, or even emotional infidelity such as flirting or sharing secrets.

To be clear: If you break the rules of sexual or emotional commitment in your relationship, whatever they may be, it is infidelity. Different relationships have different rules. You know when you've breached them.

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