Infidelity is one of the biggest causes of breakups — and of divorce.

For New York Times best selling author and “love life strategist” Matthew Hussey, cheating is “whatever the two of you have decided is beyond the boundary of what is OK.”

“There can be no one definition of cheating,” he told Business Insider. “It's whatever it is to each partner. In an open relationship, this could be that one of you slept with someone and didn’t tell [the other].”

The bad news is that cheating appears to be more common than you may have realised.

A 2015 YouGov survey for The Sun found that one in five British adults had had an affair, while a third said they had thought about it. A quarter admitted to having had two affairs, while 20% had had three or more, and 8% had had five or more affairs.

According to Hussey, though, there are three types of people that are more likely to cheat than others.

1. Narcissists.

“For the narcissist it's all about proving how lovable they are,” said Hussey. “It's not enough one person loving them; they want to make sure that everyone loves them.”

“They'll only feel validated when someone else flirts with them, sleeps with them, or kisses them,” he added.

A study found thatnarcissism is linked to people feeling less committed overall in ongoing romantic relationships, in part because they like the idea of finding someone better.

2. Insecure people.

“Insecure people are something to be careful of. And I don't mean a basic level of insecurity that we all have, because we all have our own self-doubts and worries about ourselves. But when someone is so deeply insecure that if you don't give them love today, they're going to find it somewhere else — that's dangerous.”

“In any relationship we have to be real,” Hussey added. “If someone goes without love for a long time, the propensity to cheat is going to increase. Over a long enough period of time if someone treats you coldly or is distant, anything can happen. But on a short timeline, if somebody is constantly making how much you love them today the condition on whether they'll cheat or not, [this is] also dangerous and it stems from ego.”

3. People who are just plain selfish.

Hussey said “selfish people who don't have a moral standard when it comes to their relationships” are the third type most likely to cheat. “They know if they cheat their partner would hate it or be traumatised by it, but their own selfish needs just take over,” he said.

“Either they have a different code of ethics in private for themselves — which is just unfair — or they're simply a hypocrite.”