Surprisingly, defining booty calls, one-night stands, friends-with-benefits arrangements and other casual sex relationships is important for psychologists, who can't seem to study relationships without defining them.

Peter Jonason, psychologist at the University of Western Sydney in Australia, focused on these 'casual' relationships in a new study published November 1 in the journal Archives of Sexual Behavior.

"There is such a range of relationships, and we tend to be loose in the terms we use to define and describe them," Jonason said.

He wanted to provide definitions that offered insight into how these relationships function. People get involved with other people for many reasons: sex, companionship, love, and even for self-esteem boosts. The results could explain why people might get into booty-call erlationships versus a one-night stand or long-term affair.

Booty call or one-night stand?

Jonason had previously studied booty calls, and found that they involve more emotion than one-night stands, but are far less 'touchy-feely' than long-term relaitonships.

In his new study, Jonason surveyed 192 undergraduate students - 124 women and 68 men — at the University of West Florida. Each participant was asked to rank how likely booty calls, friends with benefits (people who have casual sex while remaining "just friends"), long-term relationships and one-night stands were to fulfill these four functions: sexual gratification, social and emotional support, a "trial run" for a serious relationship, and a placeholder to stave off boredom or to bide time until something better came along.

After answering these questions, the participants filled out a survey that made them match one relationship to one function, following which they answered questions about their own attitudes toward sex and relationships to measure their individual comfort with, and interest in casual sex.

The result revealed that different relationships served different functions. The predominate function of long-term relationships was social support, followed by being a trial run or a placeholder, with sex bringing up the rear.

Booty calls were primarily for sex, with placeholder coming in as the second function and trial run coming in third. Social support came in dead last.

"Individuals engage in these different relationships to serve different needs," Jonason told LiveScience.

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