Cheaters may seem unpredictable but a new study claims to have found “the danger age.”

The magic number for when your partner is most likely to cheat is 39 – according to one study.

The findings come from a study of 1,000 people by a U.K. dating site for married people called IllicitEncounters. The study is not scientific or up to peer-reviewed standards but one expert said the age makes sense.

“I think 40 is a big deal for people. They start really reflecting on not only age but ‘Is my life meaningful?’” Nicole McCance, a Toronto psychotherapist and relationship expert, told CTV News Channel Friday.

“They’re acting more recklessly and impulsive.”

The study found men and women are more likely to cheat as they enter a new decade. The second most popular age to cheat is 49, followed by 29.

“Beware of the figure nine – it is the danger age as far as affairs are concerned,” spokesman Chris Grant said on the IllicitEncounters blog.

Partners are more likely to develop a wandering eye during the summer, McCance said.She said there are more opportunities to go out, a desire to meet new people and have fun.

“Divorce is highly prevalent in the summer because the kids are not in school,” she said adding that parents prefer to make a big transition at that time.

McCance has seen people cheat at all ages but when it comes to sites targeted at married couples, she noticed two key traits: low morality and risk-takers.

The top reason for cheating was boredom in the bedroom, which was cited by 76 per cent of study participants. It also has a lot to do with opportunity, which half of study participants listed as a factor.

McCance said cheating is about a lot more than sex: “My clients tell me things like ‘I’m really lonely in my relationship’ and ‘I don’t feel like I’m a priority.’”