PEOPLE in a committed romantic relationship actually avert their eyes from attractive members of the opposite sex without even being aware they are doing it, a study has found.

This unconscious attentional bias probably evolved to help men and women stay in monogamous relationships, which tend to have a reproductive advantage in humans, researchers at Florida State University say.

In a series of experiments, the team of psychologists found participants who were married or living together monogamously turned their attention away from images of attractive members of the opposite sex.

Immediately before being shown the faces, they were "primed" to activate or elicit mental pictures associated with mating.

The lead author, Jon Maner, a psychologist, said the reaction of looking away was automatic and occurred within the first few hundred milliseconds of seeing the good-looking person. In contrast, single participants automatically responded by increasing the attention they paid to attractive targets of the opposite sex.

Such mechanisms help people maintain their long-term relationship and resist temptation, the researchers suggest.

"This whole research area is guided largely by an evolutionary perspective. These biases have been built into our psychology to enhance people's reproductive success," Dr Maner said.

The research was published in the Journal of Experimental Social Psychology.