Being friends does more for long-term survival of a couple than anything else.
That is the finding of US psychologists, who say a close friendship is linked with both a sexually and emotionally satisfying relationship.
Laura VanderDrift, assistant professor of psychology and one of the authors, said finding a partner who was "first and foremost a friend" was a good predictor of the long-term success of a relationship.
"We found that valuing the friendship component of one's romantic relationship is important," she told The Independent.
"It leads to better relationship outcomes over time, including a more satisfying relationship, a better friendship with one's partner, and even greater sexual satisfaction."
The study, carried out at Purdue University in Indiana and published in the Journal of Social and Personal Relationships, was the first to link friendship to sexual outcomes to this extent.
It surveyed 184 people who had been in relationships for 16 months or more and asked them what they valued most about their relationship.
Among the options in the paper, which was called "On the benefits of valuing being friends for nonmarital romantic partners", were companionship, friendship, sharing new experiences together and sex.
Those couples who said friendship and companionship were the most important factors had the highest satisfaction rates for the sexual side of their relationship.
Dr VanderDrift said that couples trying to improve their relationship could solve several problems at once simply by focussing on being friends.
She advised having more shared interests and "friendship-type experiences".
"For people seeking help for their relationships, this could mean that focusing on the friendship shared in the relationship would benefit other areas of the relationship," she said.
"For seeking a relationship, this suggests that finding a partner who is first and foremost a friend could confer myriad benefits."
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