Poets wax lyrical about it, musicians lament the loss of it and dating agencies make fortunes from it.
Now scientists claim to have discovered the key to finding love, and it's simple: if you fancy someone, just tell them.
Previous studies have highlighted the importance of natural beauty and physical features, including facial symmetry and voice pitch. The latest research from Aberdeen University, however, concludes that "social cues" – someone's efforts to show how much they like a person, be it making eye contact, smiling, or a verbal "come-on" – play a vital role in the blossoming of romance.
"Our research highlights how social cues, which signal the extent to which others are attracted to you, also play a crucial role in attraction," said Ben Jones, a psychologist and one of the authors of Integrating Facial Attractiveness And Cues Of Social Attractiveness, which is published in the journal Psychological Science.
In an experiment carried out at the university's face research laboratory, 230 men and women were shown four flash cards, each depicting a face with a different expression.
These included a person making eye contact and not smiling, someone not making eye contact but smiling, not making eye contact and not smiling, and making eye contact and smiling.
"What we found was that the preference for the attractive face was much stronger when people were judging those faces that were looking at them and smiling," said Dr Jones, who co-authored the report with Lisa DeBruine.