Lead author Julia Heiman, director of The Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University, and colleagues questioned more than 1,000 couples ages 40-70 from the United States, Brazil, Germany, Japan and Spain, who were either married or living together for a minimum of one year -- but overall the couples were together an average 25 years.
The study found Japanese and Brazilian women were more likely than U.S. women to report being satisfied sexually.
"We know from other research that being in a long-term relationship has some value to health," Heiman said in a statement. "Perhaps we can learn more about what makes relationships both sustainable and happy."
The study, published in the Archives of Sexual Behavior, found men said cuddling, caressing, kissing and tenderness were important ingredients for long-term relationship satisfaction, while women were more likely to report deriving satisfaction from their sexual relationship.
For men, relationship happiness was more likely if the man reported being in good health. A partner experiencing orgasm predicted happiness for men, but not women.
Both men and women reported more happiness the longer they had been together, as did those who scored higher on several sexual functioning questionnaires.