There have been many studies debating whether it's "normal" for human beings to remain with one partner for a lifetime. Certainly, many people have done so and continue to do so. But when it comes to evolution, monogamy hasn't always been the rule of thumb. Most animals are programmed to mate with as many partners as possible, in fact. Only about 3-5 percent of mammals form monogamous bonds in the animal world. Over time, the monogamous relationship evolved in order to protect offspring from being killed by other males, scientists theorize. Humans are most closely related to chimpanzees and the bonobo ape, both species that do not restrict themselves to monogamy. The female bonobo is receptive to multiple male partners when ovulating, and males fight one another, as they do in other species, to mate with females. Males also produce more sperm than other primate species to increase their chances of fatherhood.
Christopher Ryan, author of Sex at Dawn, told a TED conference that human sexual behavior is comparable to our ape brethren. Chimps and bonobos don't restrict sexual activity, and they use it much in the same way as humans, as a manner of bonding. A new study finds that the hormone oxytocin is at the root of our desire to be with one person. Men who were spritzed with the hormone and showed pictures of their romantic partners showed a renewed attraction for that person and less so for strangers. The release of oxytocin triggers dopamine, a hormone that influences reward, motivation and even addiction. It makes people feel more trusting and more social and is triggered by massage, hugs and other social interaction. So it's not a question of what's "natural" – it's simply a question of you and your partner's desires.