Think back to the times when humans planted first seeds. And for a while, they probably attributed their harvest to divine intervention, alignment of stars or a curse of the neighboring tribe. But with time, we learned that a plant needs the right mixture of sunlight, water, temperature and nutrients and so we have solved one of our basic physiological needs for food. But why is it that we still cannot align our expectations with reality when it comes to fidelity and monogamy?  Studies show that we are still in stone age, believing in fairytales about prince charming and love everlasting, while living season after season of Sex in the City.

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“Not in my backyard!”

While infidelity often comes as a shock, it is not always viewed as a completely impossible occurrence. Several scientific studies have investigated how partners estimate the chances that their partner would ever cheat on them. One study on couples married less than ten years suggests that early on in their marriages, people estimate that the chance of their spouses ever engaging in extra-marital sex as 7,9 percent (ref.2). More than a third of people married less than ten years had complete and total faith in their spouse, reporting that there was no chance their partner would ever cheat on them (ref.3). Another study asked the same question to newlyweds, who estimated that there was a less than 3 percent chance that their spouses would engage in either a one-night, a brief affair or serious affair within the next year (ref.3). Apparently, the honeymoon phase applies to trust as well!

Officially tying the knot might be a significant gesture for some, but its effect on partner trust is not immediately clear. Individuals in committed dating relationships still mostly trust that their partners won’t cheat on them, with their faith in their partners at the same level as than their married counterparts. Those in dating relationships estimated that the probability of their partner cheating was between 5 and 9 percent (ref.1), which is within the same range as estimations found in married relationships.

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“The grass is greener on my lawn!”

When it comes to relationship fidelity, the grass is apparently not as green on the other side of the white picket fence. An interesting finding comes from one study that asked individuals about the likelihood of cheating among other relationships outside of their own. Respondents estimated that 42 percent of members of the opposite sex cheat on their partners (ref.1). This finding shows that although people significantly underestimate their chances to be cheated on, they see infidelity as reasonably common. We are quite convinced that others cheat, yet are also convinced that our own partners are different.

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And the reality…

What is the actual rate of infidelity? That data on infidelity is often hard to obtain, possibly due to the shame and embarrassment attached to the issue. Estimates greatly depend on the type of methodology employed, the sample surveyed, and the definition of adultery used by the researchers. Compiling many studies, however, we see a pattern emerging, which is significantly higher from the expectations.  Studies report from as low as 26 percent to as high as 70 percent for women and from 33 percent to 75 percent of men cheat (ref.4, 5,6). While stereotypes tend to tell us that men are more likely to cheat, studies show otherwise. Due to the fact that men generally tend to overreport and women to underreport their number of sexual partners, the rate of infidelity between genders is more or less the same.

While the studies aren’t clear on the exact rate of infidelity, what’s clear is that the rate of infidelity is much higher than what individuals expect in their own relationships. Some authors even suggest that the number of married couples that experience an affair at any point may be as high as 76 percent (ref.7).

With knots come tangles, and that includes the knots we make on our wedding day. A majority of people tend to seek committed relationships and disapprove of infidelity in marriage (ref.8), even if infidelity is quite widespread. I think that humans may not be the monogamous species that we often view ourselves, we may simply be more idealistic. In any case, as a scientist, I believe in the value of knowing. And also there is some comfort in not knowing.