Current studies of American couples indicate that 20 to 40% of heterosexual married men and 20 to 25% of heterosexual married women will also have an extramarital affair during their lifetime.
So states Helen Fisher, a biological anthropologist, Chief Scientific Advisor for the dating site Match.com and author of five books on love, sex and relationships, in a TED talk/article entitled 10 Facts About Infidelity.
Infidelity, she asserts, is part of our ancestry as prehistoric men and women found reproductive and supportive need for it–a kind of hedging your bets strategy to insure sexual procreation and survival of infancy past the first year.
A lesser known but more intriguing fact in my mind, however, it is part of human biology. The brain, she explains, is a three-part system controlling sex drive, romantic love and “partner attachment” that makes it “possible to express deep feelings of attachment for one partner, while one feels intense romantic love for another individual, while one feels the sex drive for even more extra-dyadic partners.”
In fact, a particular gene may be responsible for infidelity, a widespread phenomenon across time and cultures:
Men carrying the 334 vasopressin allele in a specific region of the vasopressin system scored significantly lower on the Partner Bonding Scale, indicating less feelings of attachment to their spouse. Moreover, their scores were dose dependent: those carrying two of these genes showed the lowest scores, followed by those carrying only one allele. Men carrying the 334 gene also experienced more marital crisis (including threat of divorce) during the past year, and men with two copies of this gene were approximately twice as likely to have had a marital crisis than those who had inherited either one or no copies of this allele.
It is always a bit disconcerting to me to read studies that nail specific behaviors ordinarily regarded as complex, affected by so many variables of time, physiology and history, to a gene. We often indulge science a great deal, affording it unquestioned authority in our everyday absorption of internet tidbits but without the benefit of perspective found in further reading on a given subject.
This article claims a gene governs the likelihood of cheating behavior, but, of course, it does not cite competing genes or other sources that govern ethics or cultural mores that influence a specific decision in any given moment. The author discusses ten facts in a bite-sized article, but are there other facts that would color the conclusions she makes?
However, for those interested in the subject, this article does reference many sources at the end of each of the “ten facts,” and for that reason and the get-out-of-fault-for-cheating free card it offers, in essence, it is a worthwhile read–food for thought anyhow.
In the gaze of the other 
I read a humanities study on women in a tribal region of Africa, bottom line: Women tended to mate outside of their bond during ovulation, perhaps nature’s way of insuring diversity of the species and, a bit more reaching but believable, this inherent infidelity being the bottom line cause for all wars since the beginning of time.
Jealousy and greed are quite the motivators of hostilities for sure.