How Do You Know Your Partner Is Cheating on You?
27 Oct 2026 · @Dr Baillieu Psychologist
You'd just know. Right?
A lipstick mark. A phone lighting up at 2am. A feeling in your stomach that something is off.
Here is the awkward truth from the research: it rarely works like that. And the biggest surprise is that a lot of people who cheat are not unhappy at home.
Stay with me. This one changes how you see the whole thing.
The telly everyone is talking about
If you have been anywhere near ITV this week, you will have felt the buzz. Adultery started on ITV1 on Sunday 25 October, and all six episodes are on ITVX.
It comes from Danny Brocklehurst, who adapted Fool Me Once and co-created Brassic, and it stars Dominic Cooper and Romola Garai.
No spoilers, I promise. The set-up is simply this: two families, two teenagers in love, and their parents suddenly feel an intense attraction to each other. Messy? Absolutely. Believable? That is the uncomfortable bit.
Dominic Cooper has said adultery is something we all question, and that everyone has been tempted at some stage of life. Whether you agree or not, plenty of us are watching with one eye on the screen and one eye on the person next to us on the sofa.
So let's do what the drama can't. Let's look at what the research actually says.
How common is it? More than you think
In a YouGov survey of British adults, one in five admitted to having had an affair. A third said they had thought about it.
Now look closer, because the details are where it gets juicy:
Men and women are almost level. 20% of men and 19% of women said yes.
Many go back for seconds. Of those who had an affair, only half stopped at one. A quarter had two, and one in five had three or more.
Nobody agrees on what counts. 22% had romantically kissed someone else, but only 17% had slept with someone else. So where does "an affair" actually begin?
It is usually someone you already know. 43% cheated with a friend and 38% with a work colleague. Only 18% did it with a stranger.
Some are still going. 5% said their longest affair is still ongoing.
That poll dates from 2015, and people are not always honest, even in anonymous surveys. Treat the numbers as a rough guide, not gospel.
Why do people cheat? It is not what you think
Psychologist Dylan Selterman and his team at the University of Maryland asked around 500 people who had cheated one simple question: why? Eight reasons came out of it, published in the Journal of Sex Research:
Anger
Self-esteem
Lack of love
Low commitment
Need for variety
Neglect
Sexual desire
Situation or circumstance (the wrong place at the wrong time)
Read that list again. Sex is only one of the eight. Here is what the findings tell us.
It is rarely just about sex. Most people felt an emotional attachment to the affair partner, and it was even more common in those who felt neglected or unloved at home. Around six in ten expressed affection towards the new person.
It is not always a broken relationship. We often hear that an affair is a symptom of a damaged relationship. Selterman says it is not that simple: people cheat for many reasons, and a lot of them say nothing about how healthy the relationship was.
Love and cheating can sit side by side. In another study, published in Archives of Sexual Behavior, almost 2,000 people who were seeking affairs reported being fairly satisfied with their marriage and feeling strong love for their spouse. Sexual dissatisfaction was the strongest motivator, and about half said they were not currently sexually active with their partner. (That sample was self-selected, so it tells us about people seeking affairs, not everyone.)
Men and women tend to report different motives. Men were more likely to cite sexual desire, variety and circumstance. Women were more likely to cite emotional reasons, especially neglect. Those are tendencies, not rules.
Selterman's warning is the line I would put on a poster: it is an unwise assumption that monogamy just comes naturally, or that if two people love each other, neither will ever want someone else.
And the plot twist? Only about one in five of the relationships in the study ended because of the affair.
Do people change completely in a relationship?
The short answer: not really. The research points less to a personality transplant and more to unmet needs, opportunity and the person someone already was.
Here is what keeps showing up:
Past behaviour. In one longitudinal study, people who cheated in their first relationship were three times more likely to cheat again.
Attachment style. People with insecure attachment are more likely to cheat in marriage.
Personality. Low conscientiousness, low agreeableness and high extraversion are consistently linked to higher rates of infidelity, and so is high neuroticism.
Biology. Twin studies suggest genes may explain some of the difference, and one dopamine receptor gene has been linked to infidelity. It is early days, so do not read this as "born to cheat".
The stories we tell ourselves. People find cheating easier when they soften the language and blame outside factors. It lets them dodge the guilt while still seeing themselves as decent.
There is a flip side too. Couples with similar personalities and values tend to be happier and to have lower rates of infidelity. And neglect and lack of love were big drivers in the Maryland study, so what happens inside the relationship still matters.
None of this is destiny. These are risk factors, not predictions about any one person.
So has your partner become a completely different person? More often the better question is: what was already there, what has gone unmet, and what opportunity turned up?
The telltale signs (and why they can mislead you)
You came here for this bit, so here it is. But I need to be straight with you first: there is no tested, reliable tell. These signs come from clinicians and relationship experts, not from a validated checklist.
With that said, these are the ones that come up again and again:
A changing schedule. More late nights at work, new business trips, or going out more often, with no good explanation.
Phone secrecy. A long new password, the screen turned away while texting, or more messages than usual.
A new look. A sudden change in appearance, grooming or routine.
Distance, or the exact opposite. Some partners pull away. Others become oddly affectionate, with flowers and "I love you" every five minutes.
A shift in your sex life. A partner who always initiated stops, or suddenly wants things they never mentioned before.
The accusation boomerang. They start accusing you of cheating. Experts describe it as a favourite tactic.
Money that does not add up. Unexplained withdrawals, or card purchases you do not recognise.
Awkward friends. Their friends seem uncomfortable around you.
Stonewalling. Changing the subject, storming off, or no longer arguing the way they used to.
Now the bit that matters most. Almost every item on that list can also be caused by stress, low mood, a hard time at work or a personal crisis. One sign on its own proves nothing.
What is more informative is a cluster of changes with no other explanation, particularly alongside a feeling that has not gone away.
What to do if your gut is shouting
First, ask yourself one question: why am I worried? Suspecting something does not mean it is happening. Name what you have actually noticed, and what has changed.
Then:
Pick a calm moment. Not midnight, not mid-row, not in front of the children.
Describe, do not accuse. "I have noticed you seem distant and I miss you" opens a conversation. "Who are you texting?" opens a fight.
Notice how they respond. Curiosity, defensiveness and flipping it back on you are all information.
Get support. A trusted friend or a therapist can help you think clearly, whatever the answer turns out to be.
And remember the plot twist from earlier: in the research, most relationships did not end because of an affair. Some break down, some limp on, and some do the hard work of repair. You get to decide what happens next.
If this made you think of someone, send it to them.
Sources
The series:
ITV's new gripping drama Adultery begins this weekend
ITV engages in adultery with Danny Brocklehurst and Poison Pen Studios (C21)
Woman & Home: Adultery cast interview
The research:
University of Maryland: Why do people cheat? Eight motivating factors
Scientific American: Why do people in relationships cheat?
Today: Infidelity study, people who cheat love their spouses
YouGov (2015): 1 in 5 British adults say they have had an affair
Amen Clinics: The neuroscience of infidelity
Bettendorf Counseling Center: Why people have affairs
AIHCP: The psychology of why people cheat
The signs:
Psych Central: Signs of a cheating spouse
Bustle: Surprising signs your partner may be cheating
AARP: How do you know if your partner is cheating?
Website: lifechanges4good.com | Email: dr.danielle@lifechanges4good.com
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