This article provides an in-depth, psychological and scientific overview to the question of why people cheat.
It explores the psychological personalities, the life events and the mental issues that leads to cheating.
But first, an overview:

Why Do People Cheat?

The short answer:

Because it’s good for them and because they can.

The slightly more complex answer:

People cheat because it provides advantages from an evolutionary point of view.
Cheating can spread our genes wider, can help us produce better offspring, or both.

Cheating is indeed a common, widespread phenomenon. And, in any large population, you don’t usually reach a wide “adoption rate” unless it makes sense for the propagation of our genetic makeup.

Example:
Max X sleeps with a woman in a relationship. Let’s call her W.
Man Y in the relationship doesn’t realize and raises the other man’s children. X hasn’t invested any time and effort, but his genes get a nice “free ride” from Y
Similarly, W might choose X because of a possibly better genetic makeup. Or simply a different genetic makeup: diversity is good for the species.

The really complex answer

The really complex answer, well…. This is what this article is about.

Evolutionary psychology provides a good overarching answer. But it’s not enough. As it’s often the case, reality is too complex for blanket answers.
When you hear things like “everybody cheats”, “monogamy doesn’t work” or “cheating is always bad”, stay away.
Let’s dig deeper then:

External Factors

A few factors are independent of personality, values and moralities. These factors don’t “force” anyone to betray, but they increase the likelihood of cheating for everyone.
Here are some of them:

Lifestyles of Opportunities

Professions and lifestyle that require lots of time away from our partners, for example:

• Regular travels
• Conferences attendances
• Military personnel in far away basis
• Nomadic lifestyles
• Long distance relationships

For most people, the workplace with its daily interactions and increased female participation, has been the main driver of the increase in infidelity in the last decades (Shirley Glass, 2004).
Road warriors and conferences, with alcohol mingling and hotel rooms readily available also lower the bar for cheating.

Example:
The movie Up In The Air is such an example:

Libertine Social Environments

We are social creatures and we have a strong tendency to accept the values that our society and social groups embrace.
And also a rather strong tendency to act along the social-sanctioned rules.

A study in The Netherlands confirmed indeed that the sanctions -or lack thereof- from the group predicted the willingness to engage in extramarital affair (Extradyadic sex: The role of descriptive and injunctive norms).

That means that if we have a lots of friends who cheat, we are much more likely to cheat. If our friends don’t cheat, or if grew up in an environment where cheating is highly sanctioned, cheating is much rarer.

Personality Factors

Some people will cheat almost no matter what. And some other won’t cheat almost no matter what. Personality is what makes the difference.

We will start with addicts.
Addictions are often relative to addictive personality, which means than addict to one thing is often an addict for another (Charlie Sheene is such an example).

Thrill Addicts

The biggest appeal for the the thrill addict –referred to as Type-T sometimes– is the danger of getting caught.
As long as they want to stop philandering, thrill seekers are actually easy to cure.

Example:
Marcello Mastroianni plays a cheating thrill seeker in Casanova 70

Infatuation Addicts

In 10 Types of Players I talked about the “infatuation players”. These are guys who are very eager in the beginning of a relationship but lose steam when the relationship gets more serious and demanding.

These guys have a pattern of “sequential monogamy”, with George Clooney being one such example. When infatuation addicts stay in a relationship long term, they are more likely to cheat to relive that initial sparkle (over and over again).

Also read:

• Signs of a player
• What to ask to uncover a player

Sex Addicts

Sex addicts seek the orgasmic release to catch that fleeting moment of satisfaction. But the release is only temporary and the cycle begins again and again.
Some of the signs of sex addicts are:

• Pornography
• Compulsive masturbation
• Massage shops
• One nights stands

Sex addicts usually show a pattern of strong sexual desire at the beginning of the relationship, but have issues integrating love and sexual attraction and often withdraw when the relationship grows more intimate.
Sometimes sex addicts even abandon marital sex.
The discovery of the affair is shocking for the spouse because she often mistakenly attributes the lack of sex to a low libido.

Addiction From Abuse

Patrick Carnes’s survey of 600 sex addicts concluded that 73% had been physically abused, and a whopping 97% emotionally abused (Carnes, Don’t Call it Love).

Love & Romance Addicts

Similar to the infatuation addicts, but these guys are in love with love and romance.
They long for the idea of love the same way a Medieval poet would have. They’re in love with a fantasy, basically.
They long for candlelight, roses and moonlight walks.

A “real relationships” with a real bond is too real for them, and that’s why they start romancing outside of the relationship.

The Starving Ego

Often the affair partners are no mo attractive than the relationship spouse. But what really attracts the unfaithful partner is the gratification and ego stroking of being loved and liked.
The starving ego is particularly sensitive to flattery and admiration.

Madonna/Whore Complex

Many men divide women into two categories: Madonnas and whores (The Moral Animal). But for some that split is so strong that it carries major consequences in their sexual lives.

The women in their relationships become Madonnas. They don’t feel they can engage in unorthodox sex with them. But they can enjoy loose women on the side.
They don’t think it’s disrespectful because the two are on two different planes anyway.
These are the type of men who would say things like:

I could never say/do that to you: you’re my wife and the mother of my children

Of course it’s a terrible mindset for the women they are with: being a Madonna comes with more donwsides than upsides.

Reluctant Grown-ups

Some people stay mentally childish in the sense that they keep refusing commitments and responsibilities well into their adult years.
And one way to escape those commitment and responsibilities is by cheating and having extramarital affairs.

Personality Disorders

Some personality disorders, like the ones in the dark triad, usually show a higher incidence of cheating. Part of the reason is the lack of morals and empathy. Morals indeed have a greater “restraining power” on cheating than even religious beliefs do (Glass, 2004).

Narcissists

Narcissists are self centered and cannot empathize with other people.
They think of themselves as special and entitled, but their self esteem is fragile and based on constant recognition from the people around.

Antisocial Types

To oversimplify, antisocial personalities are narcissists on steroids.
They do not accept social moral and values, including legal boundaries. They can be very manipulative and exploitative in relationships, and cheating is one of the ways they fail to live up to standard moral code.

Chronic Liars

Chronic lying is different than the situational lying of denying an affair.
The Chronic liars make up stories, lie on tax returns, shift blame and make up exaggerated stories sometimes that few ever believe to be true.
They have no guilt of cheating and will make up all kind of stories to cover their tracks.

The Entitled Cheaters

Some people feel entitled to having multiple partners. Some of them are:

Abusers

Abusive partners, especially men, have a sense of superiority on their partner. They feel they can get away with whatever they want, including extramarital affairs.
It’s not uncommon for abusers to fly into rages when the partners confronts them about infidelity. Even if it’s true.

Revenge entitlement

Some people who become rich and successful later in life feel entitled to enjoying multiple women to make up for years of geekiness and invisibility to the opposite sex.
Sometimes they think of themselves as committed to their relationship and would never grant the same freedom to their women.

This is mostly true for men, but in couples with high status woman and low status man -which are rare anyway-, women also end up having more affairs than men (Adultery: An Analysis Of Love And Betrayal).

Highly Successful & Driven Men

Some highly successful and driven men see themselves as entitled to success, money, authority and… Women.
In evolutionary psychology terms: the more resources one has to spread, the more mates he can attract.
Jan Halper found indeed a correlation between income and likelihood of cheating (Quiet Desperation: The Truth About Successful Men).

Usually women tolerate their philandering because of the unwritten rule the marriage entails: luxury for freedom. Contemporary examples are Silvio Berlusconi and partially Donald Trump.

The Indulged Child

Not every entitled cheater is rich and successful. Some adults are ordinary, but were brought up by their parents as if they were special and destined.
Some of them had indeed some trait that set them aside: beauty or a special talent for example. These “princesses” and “princes” expect now their spouses to grant them all the freedom without the need for reciprocity.

Family Influences

How we grew up has a huge influence on how we are. Including on how we are when it comes to fidelity.

Life Father, Like Son

Studies show that affairs and cheating is more likely to occur among individuals who have an history of affairs in their own family (Emily Brown, Patterns of Infidelity).
And the correlation is quite huge: Bonnie Eaker Weil recorded the data in her clinical experience and found out that 90% of unfaithful patients had at least one unfaithful parent (Bonnie E. Weil, Adultery: The Forgivable Sin).

Parents’ Bad Influences

Some influences are unconscious, deeper, or darker:

• Uncaring parents

Some parents are very hands off and give their children all the freedom they want. Some children can take it as a sign of neglection.
If their partners also grants them all the freedom, they might act out more and more to get reprimanded -a sign of caring-.
Eventually, they might end up cheating as their ultimate effort to get a sign of love.

• I won’t be cheated on: strike first

Sometimes a woman who sees a womanizing father make her mother suffer might vow, consciously or subconsciously, to never be like her mother.
Which means, she’ll be like her cheating father…

• I won’t be dependent like dad

If we see a parent who is stuck in a terrible marriage they have little control on, we might subconsciouly avoid any strong tie.
Cheating then becomes a way to assert our distance and independence.

Example:
The movie Blow provides such an example

• Childhood abuse

Men or women who were sexually abused during childhood may engage in compulsive sexual behavior. It goes for both men and women (Carol Rinkleib Ellison, 2000).
The movie Mysterious Skin is a good example but it’s too heavy and I won’t post it here.

• Emotional incests

Some father snub their wives and shower their daughters with gifts and attention (Patricia Love, The Emotional Incest Syndrome).
Some other times the daughter sees the father giving all the attention to a lover and not to her mother.
These girls develop the unconscious desire of being the other woman and go after married men.

• The caretaker

Some children grow up in an environment that forces them to take care of several members in their family. They give, give, give and take little or nothing.
Most of the times, it’s women.
These women tend to get stuck with cheating men who give them very little in return.

Female Homewreckers

As much as there are several types of philandering men, there are several types of women who seem to look for taken men to cheat with.

Women in Competition

The antagonist woman has not sense of identification with her gender and sees other women as rivals and enemies.
Taking their men is part of their competitive mindset.

Antitraditionalist

The antitraditionalist scoff as the institution of marriage. And since marriage makes no sense, while should she restrict herself to single men?

The Sexy Nymphet

A girl who’s grown with the subconscious drive to be the other woman will often present a facade of high sexuality to lure the men in.
Deep down she longs for more, but when she seeks a deeper connection, men stick with their relationship.

Relationship Phases

Some transitions can create discomfort and require some adjustments. The more the couple has difficulties readjusting, the more cheating is likely to happen.
These are the major phases:

Resisting Commitment (young relationship)

The couple hasn’t fully tested yet their compatibility. If they were wrong and living together proves difficult, partners can start being more open to extramarital affairs.
And of course, some partner can find solace in cheating when they are afraid of commitment.

Run Before It’s Too Late (pregnancy)

The man can feel he’s about to getting trapped for good when the woman is pregnant. Cheating at this point is a way to break free and re-assert his freedom.

Baby-Envy (parenthood)

The baby usually gets the lion’s share of the woman’s attention when he comes into this world. And hormonal changes happen.
Here are some common issues:

• Some men are less attracted to their women (see Madonna/Whore complex)
• Many women have less sex drive after a baby
• Some men are jealous of the mother/baby bond and feel left out

Respite Cheating (turbulent teens)

When children become teenagers the household can experience some turbulent times, and marital satisfaction is often at its lowest during this period.
Cheating and having an affair can provide some respite. Also, some mother can be reminded of their sexual yearnings from their own children.

Example:
The movie A Walk on The Moon presents such a case:

Should I Also Go (launching)

When children leave home, another critical phases rushes in.
Couples who experienced friction with their children can experience a re-blooming of their relationship.
But child-centered relationships where children where the glue can come apart. Or stay together while looking to outside sources of pleasure.

I Can Still Do it (retirement)

Retirement is another major adjustment period. Some can feel a pang of meaningless, wondering if there’s nothing more to live for. And an affair can give life the sparkle.
For some others, especially men, it serves to prove they are still young enough to find a mate.

Personal Troubles

As much as the relationship goes through phases, so do the people in those relationships. Cheating is more likely to happen during these phases.

I’ve Changed… And You?

Sometimes we evolve but our partner stays the same though.
That’s when cheating with a partner who is more like we are becoming is more likely to happen.
Some other times it’s not about how we are becoming but how we would like to be.

Example:
We don’t know if Mrs Doubtfire cheated, but it’s the typical example where an affair is likely to happen because of how apart the couple grew:

Midlife Crisis

Midlife crisis can bring a host of issues, including:

• Existential crisis: is this is all there is?
• Parents death: are we next? Am I doing the best I can do?
• Am I still young enough? A younger mate can prove that
• Comfort level: not much more to strive for

Example:
The lovely movie Lost in Translation is an example of an affair during middle life crisis:

Attachment Styles

There lots of good articles here on how attachment styles influence relationships and fidelity.
Here’s quick lowdown:

Secure attachment types tend to have one relationship at a time, create a strong and solid emotional bond and tend to cheat less. The monogamous types we talked about in signs your partner is cheating are usually secure attachment types.

Anxious attachment types crave emotional closeness but are always afraid their partner might not feel the same.
They often end up in a relationship with the avoidant types (Amir Levine, Attached), which makes them feel starved for affection. They might cheat to get the emotional intimacy they crave, and they get attached very quickly to the affair partner.

Avoidant attachment types are fearful of emotional intimacy. For these types an affair provides a way to get more autonomy, and researches proves avoidant cheat the most: twice as much as all the other attachment types. And they are more likely to cheat multiple times and to have more one night stands (Allen, Baucon, 2004).

Example:
Mastroianni in La Dolce Vita is a great example of a cheater-avoidant attachment type

Relationship Issues

Cheating can be the consequence of troubles in the relationship. And when you can fix those relationship issues, you can also lower the chances (and the need) for cheating.

Cheating to Escape

Sometimes life in the relationship can become a grey doldrums. And sometimes it can be straight up painful.
Imagine having to attend to a sick parent, or a child who’s had an accident.
These tragic occurrences can take a huge toll on the couple. Cheating can be a way to escape the emotional pain: the lover is the antidepressant.

Power Struggles: Cheating to Get Even

Some spouses can feel smothered under the tight grip of their more powerful partner. Cheating can be a way to getting back at him and getting even.
You fix this issue by accepting your partner’s influence

Loneliness: Cheating for Emotional Starvation

Some couples spend years or even decades without ever really getting to know each other. When they finally meet someone who listens, asks questions and wants to know their fears and dreams, then they are liable to fall for them hard.

Example:
There are many examples in movies, but I like The Good Girl which is basically all about emotional cheating:

You fix this issue by asking question, listening ampathically and getting to know your partner (Love Maps, in Gottman’s parlance). And by learning emotional intelligence and emotionally tuning in to your partner.

Child-centered Relationships

Some relationships puts the children above everything else. That also means though that there’s no space for romance between the spouses.
And when there’s no romance at home, partners look for it outside.

So Bad There’s no Escape

Sometimes people cheat at the end of the umpteenth terrible argument, or when they just can’t take it anymore and they deeply want to move on.
They can cheat as a way to “force themselves” out of the relationship or to officialize that it’s over.

SUMMARY

To simplify cheating, imagine it like this: when an individual ripe for affair meets opportunity.
The “readiness” is individual by many factors and we analyzed most of them in this article.

Sources:
This guide references the work of infidelity experts such as Shirley Glass, author of “Not Just Friends“, empirical researches quote throughout the article, evolutionary psychology sources (such as The Selfish Gene) and the author’s knowledge acquired through personal experiences and delving into psychology and relationship literature.  

More theories:
There are also some mathematical theories out there, like the CL-ALT which postulates people compare their relationships to what else they can get. It makes a lot sense, that’s how we pick mates in the first place. But I don’t believe they add much value when it comes to cheating. The rational framework of the homo economicus has long been disproven.