People often believe cheating is solely caused by lust, a need for validation, or having a few too many drinks.

Renowned psychotherapist Esther Perel, after 45 years as a sex therapist, has revealed that she finds one primary reason why people stray in their relationships.

In a conversation with The Telegraph, Perel attributes it to the “deadness” of a relationship, which can lead a partner to start developing wandering eyes.

The 66-year-old author of the highly esteemed book, Mating In Captivity: Unlocking Erotic Intelligence, and the podcast, Where Should We Begin?, notes that infidelity has existed since marriage was invented and occurs for numerous reasons.

“Sometimes those have to do with the relationship – sometimes it’s rejection, betrayal, disconnection, or alienation. Loneliness is a big one,” she explains. “Other times, the reasons are internal and have little to do with the relationship itself.”

But she emphasises that the primary cause is the “deadness,” which can occur when a couple’s intimacy has “smothered” their passion.

Or it might be that their lack of presence has convinced their partner that they no longer care.

She believes that the key to rekindling a relationship where someone is considering or has engaged in cheating is “aliveness” – something that can be sparked by curiosity and a concept she calls “differentiation.”

“It means exploration, discovery, active engagement with the unknown, and being alive. It’s a good entry point,” she says.

“It’s about opening yourself up to the possibility that you don’t know your partner as well as you think,” she adds. “Instead of settling into a cosy routine, why not have a conversation about something interesting beyond what we want from each other?

“Do you know how many people go out with friends, see their partner talking about a movie or band or experience, and then in the car or on the train, they ask ‘Who is picking up Johnny tomorrow after school?’ or ‘Did you get the groceries?’ From there, not wanting to have sex or feeling distant isn’t far behind.”

Perel also emphasises the importance of incorporating “play” into your relationship.

This can involve taking risks, sharing interesting stories, creating new rituals, or trying different foods.

“It’s about giving your partner the chance to see you and themselves in a new light,” she explains.

And when asked what sustains a long-term relationship, she stresses that not trying to change your partner is crucial.

“People often say ‘I want what is important to me to be important to you too’,” she notes.

“You can’t make someone be exactly like you. You accept your differences and understand that someone can do something for you that means nothing to them, and you can do things for your partner even if you’re not interested yourself. You don’t have to change, but you should try to accommodate.”