Proof love at first sight exists
People decide what kind of relationship they want within minutes of meeting, a study in the Journal of Social and Personal Relationships said.
Researchers at Ohio State University paired off 164 students, focusing on same-sex friendships - but said it could be applied to dating.
Report co-author Artemio Ramirez said it suggested speed dating had value as people did not want to waste time.
"It's almost a self-fulfilling prophecy. We make a prediction about what kind of relationship we could have with a person and that helps determine how much effort we are willing to put into developing a relationship.
"If I think we could become friends, I'll communicate more, tell you more about myself and do things that will help ensure a friendship does develop.
"If I have a more negative prediction about a future relationship then I will restrict communication and make it harder to develop."
Professor Ramirez, who conducted the study with Michael Sunnafrank of the University of Minnesota, said it contradicted previous assumptions.
"Earlier research had assumed there was a cumulative effect that happens in the first days of meeting that helps determine how relationships will develop.
"But we're finding that it all happens much sooner than that - literally within a few minutes."
After the first meeting, which lasted between three, six and ten minutes, the students completed a questionnaire which asked them to predict how they envisaged a future relationship developing.
They also stated how much they had in common and how much they liked the person they had just met.
Nine weeks later the participants were asked what kind of relationship had developed.
People who rated the potential relationship more positively tended to sit closer to their partner during class and talk more to that person.
After nine weeks, they were more likely to have developed a close relationship, the study found.
The results were the same for people who talked for three, six or ten minutes.
Prof Ramirez said: "That tells you things are happening very quickly. People are making snap judgements about what kind of relationship they want with the person they just met."