It's no secret that the sexual activity of married couples starts out strong and subsides over time, but researchers from three universities in the southern US say couples whose marriages last longer than half a century experience a rebound.

"As people age, they tend to be more even-keeled, which may help cut down on marital conflict and facilitate regular sexual activity into advanced age," says author Samuel Stroope, Ph.D., an assistant professor of sociology at Louisiana State University.

The research team analyzed the marriages and sexual outcomes of 1,656 married adults between the ages of 57 and 85 and, while their findings come as a pleasant surprise, they say couples that made it through 50 years of marriage are few.

"Additionally, the study used a snapshot in time -- and therefore cannot prove that length and order of marriage caused sexual frequency," says Dr. Stroope.

Yet the results are of intrigue and potential importance to the aging population, which is rapidly growing in developed countries around the world.

Dr. Stroope explains the phenomenon of the sexual rebound as stemming from trust that builds over time and the increasing confidence that develops on the part of each individual as a result of knowing their marriage is unlikely to break down.

Those who stay married to their first spouse have more sex than those who remarry, although the researchers say marriage order does not affect pleasure and satisfaction.

The study, which was published in the journal Archives of Sexual Behavior, builds evidence against stereotypes saying that seniors don't engage in sex very often.