“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.”

via 10 facts about infidelity | ideas.ted.com.