ENLARGE

Jan. 26, 2015 1:11 p.m. ET

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% 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.

The more broadly infidelity is defined, the more common it is. The number people seem most interested in is how often married people have sex with someone other than their spouse. Most studies show that between 1 in 5 and 1 in 4 married people will admit to having engaged in sexual infidelity, says Justin Lehmiller, a Purdue University psychologist who studies sex and relationships and is the author of “The Psychology of Human Sexuality.”

Yet experts say almost everyone has thought about cheating on a spouse at one time or another, whether it’s fantasizing about a date with Bradley Cooper or flirting with a colleague over lunch.

Have you ever wondered if you’re in danger of being unfaithful? The experts advise you to look at these risk categories. People who engage in infidelity typically fall into more than one.

Gender

If you are a man, you’re more likely to cheat. “Testosterone is a risk factor,” says Kelly Campbell, a psychologist and associate professor of psychology and human development at California State University, San Bernardino, who studies infidelity.

The gender gap isn’t as big as it used to be, Dr. Campbell says, and the broader the definition of infidelity, the smaller the gap. Compared with previous generations, women today have more options, more power and more disposable income. (Affairs can cost money.) The Internet makes it easier to meet someone new.

Studies show men and women tend to cheat for different reasons. In 2007, researchers at Chapman University in Orange, Calif., and California State University, Los Angeles, analyzed results of a survey of some 60,000 people—heterosexuals, homosexuals and bisexuals—asking them more than 100 questions on infidelity. Overall, the men who cheated said they did so because they were dissatisfied sexually.

“They were looking for sexual variety and excitement,” says David Frederick, an assistant professor of health psychology at Chapman and the study’s lead researcher. The women overall said they cheated because they were emotionally dissatisfied. “They were more likely to fall in love with someone else or to look for reassurance that they were still desirable,” Dr. Frederick says.

The study also found that men were more upset at the idea of a partner having a sexual affair, and women were more upset at the idea of a partner having an emotional affair—a finding that was published online in January in the Archives of Sexual Behavior.

Age

Not only 9-enders are at heightened risk of cheating. People in middle age are at lower risk because they have less time and spare energy, Dr. Campbell says. “Between about age 35 and age 50, people tend to be focused on careers and child-rearing,” she says. “You have a greater chance of cheating when you’re younger or older.”

Opportunity

Some researchers call this “environmental” risk. Are you typically around attractive people who would make high-quality alternative partners? Living in a city, spending long hours in a workplace and traveling frequently without your spouse all put you at a heightened risk for infidelity. The same goes for working closely with others one on one.

A promotion or career advancement can put you at risk. “You will have more opportunity,” says California State’s Dr. Campbell. “You can take people out to lunches and pay for hotel rooms.”

History

Once a cheater, always a cheater? Not necessarily, experts say. A lot depends on why someone cheated in the first place. If the reasons for cheating are more about the individual than the relationship, the person is at risk for cheating again, Dr. Campbell says. If the relationship was more to blame, the risk of repeating isn’t as great.

What if one or both of your parents engaged in infidelity? If you witnessed the pain a parent’s affair caused, you might be careful to avoid that behavior.

Then again, you can inherit a personality trait that predisposes you to cheat, such as a propensity to take risks. And if your parent cheated, you may believe infidelity is the norm. It is even possible you saw an upside to the affair. Perhaps it helped mom get out of a bad marriage.

Relationship Dissatisfaction

Research shows this is a major risk factor. Yet many people who are unhappy in their relationship don’t cheat, experts caution, and many who do cheat have other risk factors as well.

Infidelity and relationship dissatisfaction work both ways: You may cheat because you are unhappy in the relationship, but cheating will make you unhappier. People who are satisfied in their marriage are unlikely to cheat, experts say.

Sexual satisfaction is an important subcategory of relationship satisfaction. When Chapman University’s Dr. Frederick surveyed the 60,000 people, he asked them to rate how closely they agreed with this sentence: “I am satisfied with my relationship with my partner.” About 40% of those who strongly disagreed had engaged in infidelity, compared with just 10% of those who strongly agreed. He got similar results when he asked about the statement, “I am satisfied with my sexual life with my partner.”

Personality

Two of the so-called Big Five personality traits show consistent links to cheating, research shows. People who score low on “agreeableness” (being kind and caring about other people’s feelings) and “conscientiousness” (being dependable and having self-discipline) are more likely to cheat, says Purdue’s Dr. Lehmiller.

Other traits linked to a greater infidelity risk include narcissism (having an overly inflated opinion of oneself) and sensation-seeking (a tendency to look for thrilling and risky activities).

And then there are the commitment-phobes, who have what’s called an “avoidant” personality style. These people are a lot more likely to cheat, says Nathan DeWall, a University of Kentucky professor of psychology whose eight studies on attachment style and infidelity were published in 2011 in the Journal of Personality and Social Psychology. “When you ask them what their thoughts are on infidelity,” Dr. DeWall says, “they say it’s not bad.”

—Write to Elizabeth Bernstein at elizabeth.bernstein@wsj.com or follow her on Facebook or Twitter at EBernsteinWSJ