“Friendship," wrote CS Lewis, “is born at that moment when one person says to another: ‘What! You, too? Thought I was the only one.’” He wasn't the only one. Plato wrote "similarity begets friendship" in his 360 BCE play Phaedrus. And Aristotle had the same idea when he wrote, "some define it as a matter of similarity; they say that we love those who are like ourselves."Friendships blossoming on the basis of similar ideas, outlooks or tastes may seem intuitive, but that intuition is deceiving. Most friendships develop between people who are not family members or sexual partners, so friendship can't be explained on the basis of genetic or reproductive interests. Instead, evolutionary biologists have typically relied on a tit-for-tat process known as reciprocal altruism to explain friendship: you scratch my back, and I'll scratch yours.The problem, however, is that social psychologists have discovered that people do not maintain mental ledgers of favours given and received. Primatologist Joan Silk described the riddle of friendship neatly: "reciprocity and equity are important among friends, but tit-for-tat reciprocity is antithetical to the formation and maintenance of close friendship. If these seemingly contradictory claims are correct, then friendship presents a puzzle for evolutionary analysis."As with any evolutionary puzzle, it makes sense to look towards the animal kingdom for clues. Recently, a group of French shark scientists looked at whether aggregations among sharks could be explained in social terms – that is, if they were friendships – or whether sharks occupied the same space at the same time simply because of overlapping home ranges or mutual food sources.Read more at BBC News.