1. Pairbonding is a hallmark of humanity. Data from the Demographic Yearbooks of the United Nations on 97 societies between 1947 and 1992 indicate that approximately 93.1% of women and 91.8% of men marry by age 49. More recent data indicates that some 85% of Americans will eventually marry.

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2. However, monogamy is only part of the human reproductive strategy. Infidelity is also widespread. 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.

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3. Brain architecture may contribute to infidelity. Human beings have three primary brain systems related to love. 1) The sex drive evolved to motivate individuals to seek copulation with a range of partners; 2) romantic love evolved to motivate individuals to focus their mating energy on specific partners, thereby conserving courtship time and metabolic energy; 3) partner attachment evolved to motivate mating individuals to remain together at least long enough to rear a single child through infancy together. These three basic neural systems interact with one another and other brain systems in myriad flexible, combinatorial patterns to provide the range of motivations, emotions and behaviors necessary to orchestrate our complex human reproductive strategy. But this brain architecture makes it biologically 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.

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10 facts about infidelity | ideas.ted.com.