This article was originally published in Le Point on 24.02.2026
Let’s say, hypothetically, that you suspect your partner is cheating on you. Maybe you’re just imagining things, or it’s your old paranoia flaring up again. Either way, your sixth sense is tingling. The question is, what would you do—or be willing to do—to find out if your partner is secretly enjoying another person’s earthly delights?
That was, more or less, the question posed to men and women in a recent study investigating “infidelity-detection strategies” in our species. The authors of this study, psychologists Menelaos Apostolou and Maria Ioannidou from the University of Nicosia in Cyprus, approached this “complex and fascinating phenomenon” through an evolutionary lens. They point out that, due to powerful unconscious genetic motives, we fallible human beings are fundamentally torn between securing long-term romantic relationships (“legitimate partnerships”) and indulging in carnal hookups on the sly (“extra-pair couplings”).
You’ve got to stand way back from any churchly, moralistic assumptions to see it this way—on a scale long before talking serpents and cursed apples were even a twinkle in the eyes of lascivious storytellers. Tens of thousands of years ago, the authors remind us, and certainly before agencies such as the Sécurité Sociale provided a life raft for the destitute, having two committed parents significantly increased the odds that children would survive to reach sexual maturity. Any shared offspring who subsequently reproduced were, of course, to the advantage of both parties. Being in an intimate long-term relationship also came with material and non-material benefits, such as pooled resources, protection, companionship, mutual healthcare, status climbing, and so on.
But monogamy had a hefty price tag, too, at least in the amoral game of life that is evolutionary biology. You’re essentially placing all your holdings in one bank, forfeiting potentially lucrative opportunities that might arise from a more diverse genetic portfolio.
When you consider that the average global ejaculation latency hovers at around just five minutes, and that men are not the ones at risk of being hobbled for the better part of a year by a potentially deadly, rapidly growing organism doing gymnastics in their pelvic cavities, nor having their teats drained for years more by this adorable cooing creature, it’s not surprising that casual sex in the form of infidelity evolved to be more tempting to men than to women.
But women, too, have clear Darwinian reasons for cheating. Potentially, it gives them access not only to additional resources, but lines up a replacement if their husband doesn’t pan out for some reason, abandons them, or suddenly dies (a near-constant threat for women in pre-industrial times). Then there’s improving the quality of their own genetic lineage. If a “monogamous” woman is surreptitiously impregnated by a man whose genes are an improvement over the genetic material of her committed partner, yet that partner raises the child thinking it’s his own (i.e., cuckoldry), this would have been—before the days of DNA testing—a highly adaptive tactic.
You can see how these endless reproductive tensions would have played out over the eons. Ultimately, Apostolou and Ioannidou suspected, a sort of intelligence arms race would have emerged between those trying desperately to detect their partner’s infidelity, and the latter trying desperately to hide their “making the beast with two heads,” as Shakespeare put it, with other people on the side. The data are striking. For example, 33% of French women say that, at some point in a long-term relationship, they’ve been unfaithful. In the US, the numbers are similar: about 30% of married men, and 20% of married women, report having had an affair. That’s just those who admit it on anonymous surveys, so it’s probably an underestimate. Another study found that up to 70% of dating (non-married) couples report at least one incident of infidelity.
So, back to that juicy question: how far would you be willing to go to know if you’re being played? In an initial study, people were interviewed and told to write down things they’d do to find out if their partner was cheating on them. This led to an embarrassment of riches in private sleuthing techniques. Forty-seven discrete acts were identified—many quite crafty. Those 47 acts were then presented to nearly 1000 men and women, most of them in their early thirties. These people were asked to rate the likelihood that they’d employ each one, if indeed they suspected infidelity.
I won’t list all the sundry tactics the participants came up with for detecting a straying partner. However, after crunching the numbers, the various approaches could be grouped into six clear themes, or supraordinate infidelity-detection categories. These are presented below, starting with the most popular strategies to those least likely to be endorsed.
Observe changes in his/her behavior (78% of sample would do this) (e.g., “I would notice if we had sex like before”; “I would notice any unusual manifestation of love towards me”; “I would notice if he/she paid more attention to his/her appearance”; etc.)
Ask and observe his/her reactions (67% of sample) (e.g., “I would pretend that I already know that he/she is not faithful to me to see his/her reactions”; “I would discuss with him/her the infidelity of an acquaintance to see how he/she would react”; etc.)
Check where he/she is (61% of sample) (e.g., “I would visit him/her unannounced at work”; “I would return home at unpredictable times”; etc.)
Search his/her things (42% of sample) (e.g., “I would try to gain access to his/her social media account”; “I would search his/her mobile”; “I would search his/her credit card bills for unjustified expenses”; etc.)
Use of friends (22% of sample) (e.g., “I would try to fish for information from our mutual friends”; “I would put someone to flirt with him/her to see how he/she would react”; etc.)
Objective evidence collection (17% of sample) (e.g., “I would use hidden cameras”; “I would pretend to be away, e.g., on a business trip, but I would stay to watch what he does”; “I would use bugs”; etc.)
Some of these strategies are obviously more invasive than others, and risky in themselves. Involving friends, for instance, could backfire, because they might tip off the partner, and now someone else out there knows there’s trouble in paradise, making for gossip fodder and threatening your social standing. Trawling through your wife’s private messages or catfishing her on her Instagram account might uncover a sordid affair, but such acts—if you’re caught—might be seen as betrayals in themselves, torpedoing the relationship even if she’s just talking about getting her nails done with the girls or how cute you looked when slurping ramen noodles. If you hire a private investigator, there may be no coming back from that. Still, desperate people do desperate things. And in fact, most participants indicated a willingness to use more than one broad strategy—nearly 60% said they’d engage in three or more.
There were, I should add, a minority (11%) who claimed they wouldn’t do anything at all. Presumably they didn’t mind being cheated on, or at least thought it was none of their business. Open marriages and polyamory may be chic. But it’s hard to see how such an insouciant, totally non-jealous attitude would be evolutionarily adaptive, reason the authors:
Individuals who fail to detect their partners engaging in extra-pair relationships would find themselves suffering considerable costs, such as losing their mates’ investment to others. On the other hand, those who are able to detect infidelity can minimize these costs, for instance, by warning their partners to terminate their extra-pair relationship . . . [Also] if individuals know that their partners can effectively deploy infidelity-detection strategies, they would refrain from engaging in extra-pair mating in the first place, because they would fear that they could be detected.
Remember that all-important arms race, however, between those being cheated on and those doing the cheating. For the former, nature favored hypervigilance and behavioral tactics that helped to detect infidelity, but the latter had their own adaptive tricks. Indeed, a follow-up study by Apostolou looked at things from the point of view of two-timers.
“Consider the following scenario,” these people were told. “You have an extra-pair relationship. Write down some things you would do to keep this relationship secret from your partner.” I realize this may be, ahem, more hypothetical for some of you reading this than for others, just as it was for those asked this question in the study. Nevertheless, simulated strategies for hiding infidelity were largely countermeasures against those in the earlier study for detecting infidelity. Some of these were almost perfect matches, in fact.
Nosy partner sniffing around your things? “I would delete everything that has to do with my parallel relationship from my cell phone and computer.” Feel like you’re being interrogated or subtly probed over afternoon tea? “If my partner suggested that I have a parallel relationship, I would try to stay calm so that nothing is suspected.” Worried he’s clocked your flailing interest in him? “I would try to keep my sexual life with my partner active so that nothing is suspected.” And so on. The most common infidelity-hiding strategy (endorsed by 94%) was be discreet, covering everything from “I would meet my parallel relationship in remote places” to “I would make sure I have credible excuses for my absences.” Not surprisingly, people high on the personality trait of Machiavellianism (those with a cynical disregard of conventional morality and more willing to lie, deceive, and manipulate others to get their own way, such as, you know, expressing love to their partners to ensure them of their devotion) were more likely to employ the full range of infidelity-hiding strategies.
Beware though. While having one’s cake and eating it too might be an adaptive strategy, the consequences of cheaters failing to hide their infidelity are arguably greater to their genetic interests than failing to detect a partner’s infidelity. Consider, writes Apostolou:
Even if the [cheating and/or affair] continues, the legitimate partner would become much more vigilant, turning further extra-pair mating more difficult. Moreover, detection of infidelity may lead to retaliation by the legitimate partner. One act of retaliation can be that the legitimate partner engages also in extra-pair mating. In addition, retaliation can involve physical violence and even homicide.
Is all fair in love and war? No, actually. At least, it shouldn’t be. It’s easy to wax theoretical about the evolution of betrayal and affairs of the heart—and loins. But like you, I imagine, I know that type of pain all too well. And speaking in such distancing language is important not just for cold academic reasons, but therapeutic ones too. It makes our pain and loneliness more understandable—more human, perhaps even a little more tolerable.
Like what you read? Toss a tip in the jar—any amount that makes sense for you—so I can keep overthinking things on your behalf.