You probably saw dozens of people’s faces today, many more if you live in a city. You may not have been conscious of it, but you were subtly judging every one by its beauty. Your eyes are drawn to more attractive faces, and the almost inescapable result is that more attractive people have advantages in almost every aspect of life, from job interviews to prison sentencing. But what drives us to crave beauty? According to one theory, gazing upon beauty stimulates the brain’s μ-opioid receptors (MOR), thought to be a key part of our biochemical reward system. At least in rodents, stimulating or inhibiting MOR neurotransmission not only tweaks the animals’ appetite for sex or food, but also the strength of their preferences for particular foods or mates. Is our preference for pretty faces driven by the same biochemical reward circuit? To find out, researchers invited 30 heterosexual men to browse a series of female faces on a computer (one pictured). Each man received either a dose of the MOR-stimulating drug morphine, the opioid receptor–inhibiting drug naltrexone, or a placebo. The results, published today in Molecular Psychiatry, suggest that we seek out beautiful faces at least in part because our brains reward us . Not only did stimulating MOR neurotransmission cause men to linger longer on faces that they rated as more beautiful, but the beauty rating also became more extreme, with beautiful faces rated as even more attractive relative to the rest of the faces. Inhibiting MOR had the opposite effects. The findings are yet more evidence that our social interactions are strongly influenced by the invisible hand of evolution, pushing us to find attractive mates. But the question remains, how do we decide which face is attractive in the first place?