After the newlywed phase of spontaneous, frequent sex is over, couples tend have less sex as the marriage continues. A new study shows that a rebound in sexual activity occurs in couples who have been together more than 50 years.

"Results showed that individuals in their first marriage had more frequent sex than remarried individuals; marital duration had a curvilinear (U-shaped) relationship with frequency of sex; and a linear relationship between marital duration and frequency of sex varied by gender such that men had more frequent sex than women in younger marriages," researchers from Louisiana State University, Florida State University and Baylor University wrote.

The three sociologists also found that couples that stay in their first marriage have more sex than couples in a second marriage.

The study included 1,656 married adults between the ages of 57 and 85, as well as data collected by the National Social Life, Health and Aging Project. Researchers noted that the number of couples to reach the half-century mark of togetherness was a small one.

"Additionally, the study used a snapshot in time - and therefore cannot prove that length and order of marriage caused sexual frequency," said former Baylor researcher Samuel Stroope in a news release from Baylor University.

Why is there a newfound interest in sex after 50 or more years together? Stroope, who is currently an assistant professor of sociology at Louisiana State University, said, "it may be that the permanency of the relationship contributes to sexual relations picking up a bit at the end."

"Growing old as a couple, with the experience and knowledge that come with that, may play a part," he said. "You are able to learn about your partner and build on that over time. You may have a higher level of trust when you feel that your spouse isn't going to go anywhere. The expectation that the relationship will continue may give you more reason to invest in the relationship - including in sexual aspects of the relationship."

What about the remarried folk? Why do those in a second (or third or fourth) marriage have less sex than they did in a first marriage? "It may be that those who have been married in the past may not have as strong of a sense of permanence or lasting investment," Stroope said.

"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," Stroope added.

This study sheds light on a subject not often researched or discussed. The paper was published in the journal Archives of Sexual Behavior.