At the heart of all long-term relationships lies a fundamental deception, say scientists.
Women stay with men for security, and men remain with women for sex.
Researchers in Holland and Sweden say in the journal New Scientist that the deception is the driving force behind the evolution of monogamy. By offering sex all the time, they say, females in monogamous species, such as humans, birds and porcupines, disguise whether they are fertile and trick males into staying. The result is that men haplessly become monogamous, unable to move on to a new partner because they cannot determine whether their most recent coupling has resulted in pregnancy. Although it intuitively makes sense, scientists had no explanation why men were mainly monogamous, whereas males in most species seek to mate with as many females as possible to maximize the number of children they father. Usually, a male choosing a stable relationship over a philandering lifestyle would have fewer offspring, putting him at an evolutionary disadvantage.