1 | He's Getting Older

A fascinating 2014 study that looked into the activity of people on Ashley Madison found that men were more likely to look for an extramarital affair if they were about to hit a big milestone birthday, like 50 or 60. As such, a man's likelihood of committing adultery tends to increase with age, especially once they hit a point at which they feel like it's their last opportunity to sleep with other women before they're officially over the hill. For more on the phenomenon, This Is the Age When Men Are Most Likely to Cheat.

2 | She Knows She's At Peak Attractiveness

A recent study of online dating behavior found that while male attractiveness peaks at 50,  the average woman's desirability drops from the time she is 18 until she is 60. Unlike men, therefore, women are actually at greater risk of committing infidelity when they are younger, as they know that's when they have the greatest pool of applicants to choose from. For more insight into this data, This Is the Age When Women Are Most Likely to Cheat.

3 | He Feels Threatened

The prevailing theory on why men cheat is that they feel threatened or insecure about their masculinity. So it may serve as no big surprise that, according to a May 2015 study, men who are 100 percent financially dependent on their wives are three times more likely to cheat than men who live in households in which they are the breadwinner. "I think it has to do with our cultural notions of what it means to be a man and what … the social expectations are for masculinity," said the study author, Christin Munsch, an assistant professor of sociology at the University of Connecticut.

4 | She Feels Neglected

"In my experience, most women cheat (or explain/justify their cheating) because their emotional needs were either not being met, or were perceived as not being met by their partner," Marilyn Williams, founder of the MEDIAN Center for Resilience and Brain Training, told the List. "They feel lonely, ignored, not paid attention to, etc. Most of the time the cheating was not premeditated; they realized how lonely or vulnerable they were once someone else started paying attention to them."

5 | He Feels He Has Carte Blanche

It's odd to think about, but the entire idea that a woman has the right to cheat is a relatively new concept in history. Some feminists even believe that the entire idea of monogamy stems from men trying to level the playing field once women gained the ability to have sex outside of a relationship themselves. And for centuries, evolutionary scientists have drummed it into our heads that men are naturally more inclined to spread their seed as widely as possible, whereas women are programmed to hold onto their prospective father of their children so that he can protect her and her kin from wild animals.

Since we no longer live in an era in which men need to shield women from wooly mammoths, this theory doesn't really hold anymore, but some more chauvinistically-inclined males still hold onto it for dear life, and it affects their behavior.

6 | She Wants Better Sex

The prevailing opinion around female cheating is that women do it for emotional reasons more so than sexual ones. But in her book, The Secret Life of the Cheating Wife: Power, Pragmatism, and Pleasure in Women's Infidelity, sociologist Alice Walker claims that many of the women that she interviewed said they cheated purely because they wanted someone who was more sexually compatible. Some even saw it as a way of saving the marriage, as finding sexual satisfaction outside of their spousal arrangement made them feel less resentful toward their husbands. For more on this, check out 15 Signs Your Wife Is Cheating On You.

7 | It's In Their Nature

Some people really are simply more likely to cheat regardless of what kind of relationship they're in. In her groundbreaking book, Why Him? Why Her?, anthropologist Helen Fisher outlined four basic personality types:

• An Explorer is driven by dopamine, and is therefore drawn to adventure, excitement, and novelty.
• Builders are driven by serotonin, so they tend to be more cautious and self-controlled, as they value rules and stability above all else.
• Directors are driven by testosterone, making them honest, confident, assertive, and analytical.
• Negotiators are driven by pre-natal estrogen, which means they tend to be trusting, nurturing, and empathetic.

Explorers are the most likely to cheat because they get bored quite easily. It doesn't help that they tend to be charismatic, smart, and talented, making it all the easier to stray. Additionally, a 2014 study found that 62 percent of men who cheated had mutations in vasopressin, a powerful hormone that effects social behavior such as trust, empathy and sexual bonding in humans and other animals. Which, again, means that some people really are just the "cheating kind."

8 | They Need an Ego Boost

Insecurity about one's own attractiveness levels is a driving force in cheating for both men and women. People who have low self-esteem are known to be at a greater risk for cheating because they crave reinforcement and getting it from one person isn't enough. People like that are less likely to leave a relationship in which they don't feel good about themselves, because they fear being alone, and are therefore more likely to use cheating as a coping mechanism for their own insecurities.

"If the partner gets to a place in the relationship whereby they are unable to fulfill that void any longer, the cheating individual continues to stay in the relationship (because they fear being alone) but gets the validation that is now missing from an extra-marital affair," Kelly Armatage, a cognitive behavioral therapist, relationship coach and speaker, told Fox News.

9 | Their Parents Cheated

It's well-known that the relationship between our parents has a strong impact on our own romantic bonds. So if someone's parent's frequently committed adultery, their offspring is often more likely to view this kind of behavior as a normal, if somewhat unsavory, part of life. In fact, a small study published in April 2015 found that people whose parents cheated were twice as likely to cheat on their spouse than those whose parents had been faithful.

10 | It's Easier Than Ever

While technology has its perks, one of its downsides is that the age of online dating has given way to an unprecedented array of options for infidelity.

11 | There's A Fear of Commitment

According to psychotherapist Charlotte Howard, some people are "scared of intimacy in a way that allows sexual desire to only be felt outside of a deep partnership, because there is too much closeness with a partner to feel safe merging through sex." To avoid being vulnerable, they place barriers with the person by never fully committing to them.

12 | They Make a Lot of Money

A 2002 University of Washington study found that people who make $75,000 and up are 1.5 times more likely to cheat than those whose annual salaries are $30,000 or less. One can postulate that this is because people who make a lot of money tend to be Explorers and the same charisma and hunger for a new challenge that gets them to the top is a double-edged sword, as it also puts them at higher risk for infidelity. Given that you don't need to make more money than that to be happy anyway, there may be a benefit to capping your salary off at a certain point.

13 | They Live in a Certain City

From the perspective of supply and demand, how much adultery you can actually manage to commit depends on your number of options. For more on this, check out The 20 American Cities with the Most Unfaithful Husbands.

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