The science of cheating: What psychologists who've spent years studying infidelity have to say about when and why a partner will be untrue

A 2015 study of about 2,800 people between ages 18 and 32, published in the American Sociological Review, suggests that a person who is completely economically dependent on their spouse is more likely to be unfaithful. 

That's especially true for a man who relies financially on a woman. About 15% of men who are completely financially dependent on their wives cheat, compared to 5% of dependent women.

Here's the really interesting part: Men are less likely to cheat the more money they make relative to their spouse — until they bring in 70% of the household income, at which point they become more likely to cheat.

Women are also less likely to cheat the more money they make relative to their spouse — but their cheating rates don't seem to go up at any point.

For a 2015 study, published in the journal Personal Relationships, men and women read about hypothetical scenarios in which their partner had sex with someone of a different sex or the same sex.

When researchers asked participants how they would feel about it, the men were more likely to be angry and more inclined to end a relationship if their partner cheated with someone of a different sex. But they were more likely to be aroused if their partner cheated with someone of the same sex.

Women also said they'd feel more negatively if their partner cheated with someone of a different sex. But they'd be more inclined to end the relationship if their partner cheated with someone of the same sex.

Relationships are bound to disintegrate — but not yours, of course!

In a 2015 study published in the Journal of Social and Personal Relationships, university students estimated that the average person of the opposite sex has about a 42% chance of cheating on their partner.

But when it came to their own partners, participants estimated that there was about a 5% chance that their partner had already cheated on them and about an 8% chance that they would cheat on them in the future.

As it turns out, 9% of participants said they'd really strayed.

A 2013 research published in the journal Evolutionary Psychology found that most heterosexual men say they'd be more upset if their partner was having a sexual relationship with someone else but hadn't fallen in love with that person.

Most women, on the other hand, say they'd be more upset if their partner had fallen in love with someone else but hadn't had sex with that person.

In 2014, researchers looked at activity on Ashley Madison, a dating site for people who are already in relationships. They came to a fascinating conclusion, which was published in Proceedings of the National Academy of Sciences. The men studied were more likely to seek extramarital affairs when their age ends in the number nine.

In other words, right before they hit the big 4-0 or 5-0, they have a greater chance of trying to find meaning in life by having a relationship with someone who isn't their partner.

The researchers observed a similar, but smaller, effect among women, according to The Wall Street Journal.

A growing body of research suggests that certain people are more likely to be unfaithful, depending on their biology.

For example, one study from the University of Queensland, published in the journal Evolution and Human Behavior, found that infidelity was more common among people who had specific types of oxytocin and vasopressin receptor genes.

As Richard Friedman, a professor of clinical psychiatry at Weill Cornell Medical College, explained in The New York Times, vasopressin is a hormone related to social behaviors including trust, empathy, and sexual bonding.

According to the results of that study, a whopping 40% of instances of infidelity in women and 62% in men had to do with genetics.

M. Gary Neuman, who developed the "Creating Your Best Marriage" video program, told Business Insider that it is possible to save your relationship after one partner's been unfaithful.

But there are certain guidelines to follow after the infidelity's been discovered, according to Neuman, including:

1. The cheater has to feel some remorse and want to change their life

2. The victim has to make sure the cheater has completely stopped cheating

3. The victim probably shouldn't ask sensitive questions about what exactly went on between the cheater and the other person

Insider's Kristin Salaky reports that emotional infidelity is becoming more common even than physical infidelity.

Salaky points to research from the American Association of Marriage and Family Therapy, which found that about 45% of men and 35% of women have admitted to having an emotional affair. That's a lot more than the 20% of people who admit to having a physical affair.

An emotional affair is hard to define, but if you suspect your partner might be having one, there are some red flags to watch out for. In her 2012 book, "Chatting or Cheating," licensed marriage and family therapist Sheri Meyers outlines some. For example, when you argue, your partner's fallback position is about your relationship ending. Or when you ask your partner about their friendship with another person, they get defensive or evasive.

New York Magazine reported that, while infidelity was once considered men's domain, it's now about equally likely among men and women.

The magazine cites a 2011 study, published in the journal Archives of Sexual Behavior, which found that about 23% of men and 19% of women in heterosexual relationships reported having cheated on their partner. 

An analysis by Nicholas Wolfinger at the Institute for Family Studies found that Americans aged 55 and older are now more likely to report having extramarital sex than Americans under 55.

That's the opposite of what was happening as recently as the year 2000, when older Americans reported having less extramarital sex in the annual General Social Survey. The GSS is a survey that has been administered regularly to Americans since 1972 by the research institute NORC at the University of Chicago.

Wolfinger submits a number of potential reasons for this growing trend. For one, people now in their 50s and 60s came of age during the sexual revolution. Older Americans have also become less disapproving of sex outside of marriage.

That said, it's impossible to explain these findings with absolute certainty.

For a 2017 study published in the Journal of Sex Research, researchers asked participants to indicate the most important reasons why a person wouldn't be unfaithful to their partner.

A total of about 400 people living in Israel were surveyed, ranging in age from 24 to 60 years old. All had been married for at least one year and had at least one child. The top four reasons to emerge were morality, the effects on children, fear of remaining alone, and effects on other people (especially the extramarital sex partner).

Interestingly, religious participants were more likely to cite morality and concern for other people as reasons for staying faithful; secular participants were more likely to cite the fear of being alone.

Business Insider's Lindsay Dodgson reports that the old adage "once a cheater, always a cheater" could be based in truth.

A 2017 study published in the journal Archives of Sexual Behavior followed nearly 500 adults through two mixed-gender romantic relationships. Researchers asked participants to report their own infidelity and whether they knew or suspected that their partner had been unfaithful.

As it turned out, participants who had reported being unfaithful in the first relationship were three times more likely to report being unfaithful in the second, compared to people who hadn't reported infidelity.

Interestingly, participants who had reported that their first partner had cheated on them were twice as likely to report that their second partner had cheated on them.

• Read more:
• Ways your relationship could change after cheating
• Relationship facts everyone should know before getting married
• How to make a relationship stronger after cheating