Geoffrey Greif and Kathleen Holtz Deal, both of the University of Maryland School of Social Work, co-authors of the book, "Two Plus Two: Couples and Their Couple Friendships," said the findings were based on interviews with 123 couples with both partners present, 122 individuals who were alone when questioned about their relationships and 58 divorced individuals.
The research found differing motivations behind couples' friendships, with some preferring to share emotions while others see the purpose as fun and recreation, the researchers said.
Deal, who has been married 43 years, said she was surprised to find she and her husband were in the minority because they set out as a pair to make friends with other couples -- how friendships begin vary, but the majority grow out of a friendship between two people that widens to all four.
Deal and her husband have established friendships with a group of five other couples for more than 30 years sharing social events and vacations.
Greif and Deal concluded healthy couple friendships make a marriage more fulfilling and exciting.
Nonetheless, the researchers found the topics of sex and money continue to be taboo even among friends.