I teach a university class on close relationships, and the last few weeks of the quarter we discussed some of the darker aspects of our intimate associations — betrayal, conflict, violence, and so forth. During our discussion of betrayal, one of the students raised his hand and asked, “Is cheating natural?” A very interesting (and intense) class discussion ensued, with some arguing yes, and others just as emphatically claiming no. We ultimately decided, based on the available scientific evidence, that the answer depends on what we mean by “natural.”If by natural we mean socially acceptable, then the answer clearly is no. Large-scale surveys conducted here and in other countries demonstrate that nearly all men and women have extremely negative attitudes toward infidelity. Most people believe that once two individuals are in a committed, romantic relationship with each other, they should confine their sexual activities to that relationship. [Now, if those individuals choose to include other sexual partners in their marriage on a consensual basis, that is another matter entirely — the research I’m discussing here is concerned only with people’s attitudes toward sexual activity that is occurring outside of the primary relationship without the partner’s knowledge or consent. Such “extra-relational” sex is not widely accepted.] Cross-cultural research tells a similar story — married men and women found guilty of infidelity are punished (sometimes quite severely) by most societies around the world.If by natural we mean commonly practiced, then the answer also clearly is no. Recent national surveys of adults living in the U.S. reveal that most people do, in fact, practice what they preach. Around 75 percent to 98 percent (depending on the population surveyed) of people in committed relationships are faithful to their partners.However, if by natural we mean a product of our evolutionary heritage, then the answer is ... maybe. Certainly as a species we are designed for fidelity. The human infant is fragile, the ancestral environment was harsh, and the only way for our ancestors to win the evolutionary game — to survive, reproduce, and raise offspring to sexual maturity — was to find an appropriate partner. Those early humans who formed relationships with emotionally stable, smart, trustworthy, sexually faithful partners are the ones who survived, reproduced, and became our ancestors. Those early humans who made poor choices — who chose capricious, untrustworthy partners who were unwilling or unable to ignore the sexual temptations posed by others — were much less likely to win the game. Thus, fidelity is part and parcel of our evolutionary heritage.At the same time, however, it remains possible that infidelity may have served an adaptive purpose in the ancestral environment, at least for some individuals on some occasions — maybe some cheaters enhanced their chances of personal survival by receiving resources (food, protection, social support) from their additional sexual partners. Maybe some enhanced their chances of genetic survival by cheating (and reproducing) with healthy, strong, fertile sexual partners whose good genes enabled them to create healthier, stronger offspring. We won’t ever know for sure, because we can’t travel back in time, but the argument can be made that both fidelity and infidelity are part and parcel of our ancestral heritage.I suspect that what my student was really asking was whether infidelity was “okay” or “right.” Science says that infidelity is not socially acceptable, that it is not commonly practiced, and that it was not the primary mating strategy of our early hominid ancestors. Whether infidelity is right, well, there are some questions that science can’t (or perhaps shouldn’t) answer. I’ll leave that one to you.