Even in the strongest relationships, there’s always a possibility of infidelity. It may seemingly come out of nowhere, but there are typically signs or red flags before a partner strays. Researchers are working to identify some common warning signs to hopefully prevent cheating in the first place.

One thing they’ve identified is an age when cheating is most likely to occur.

They do caution that this is just a theory, but there seems to be an age when most men and women choose to find a new partner outside of their relationship.

In looking at the website Illicit Encounters, researches examined many attributes from the users’ profiles. Although they acknowledge that these profiles may not be completely accurate, one thing jumped out at them regarding the users’ ages.

They found that a plurality of the people on the site had an age that ended in nine. These were people aged 29, 39, 49, etc. Researchers developed a theory based around these ages, showing why they may lead to people cheating.

As people approach a new decade, their mortality begins staring them in the face.

Men may begin to worry that they’re becoming less attractive to women and therefore losing their virility. Finding a new woman shows these men that they’re still masculine, even as they age.

Women, on the other hand, may begin to feel unsatisfied with their partner or worry that they aren’t attractive. A new partner can suddenly provide the satisfaction that their relationship can’t bring.

Researchers also found that income is correlated with cheating.

Men who make less than their wives are 15 percent more likely to cheat. Men who make as much as their wives are five percent more likely to cheat.

They theorized that this is due to the societal pressures men have based on traditional gender roles. When a woman makes more than a man, it may be emasculating to him and threaten his manhood. Cheating on his wife can redeem his masculinity to the rest of society—at least in his eyes.

Although these theories are intriguing and make some sense, they may not tell the whole story.

The truth is that human relationships are extremely complex and can’t be fully explained by any single theory or any single behavior. There are some truths uncovered by this research, but it’s far from a complete answer as to why people cheat.

Researchers do say that there’s one common theme in every infidelity: opportunity. If a man has no opportunity to cheat, he’ll remain faithful. If he suddenly has the opportunity to have an affair, his honesty and trustworthiness will be tested.

If you’re in a relationship where you worry your man may be tempted to cheat, that’s a major red flag. The same goes for women.

In the end, it may be more likely that a partner will cheat if they’re worried about their progressing age and feel the need to prove something. If they have those nagging feelings and suddenly have the opportunity to cheat, they may stray. It won’t matter what age that person might be, if they’re not satisfied with themselves, they may seek to prove themselves by cheating.