Facial attractiveness is a powerful cue that affects social communication and motivates sexual behavior. 1–3 Attractive people are both judged4 and treated5 more positively, reflecting the biased stereotypical notion that ‘beautiful is good’. Indeed, beautiful faces are processed by the limbic reward system6 and according to the same economic principles as non-social rewards. 7 The human reward system has a high density of μ-opioid receptors, 8 which have an important role in affiliation and attachment. 9–11 Here, we causally test whether the healthy human opioid system mediates facial attractiveness preference. In rodents, μ-opioid (MOR) neurotransmission can increase both hedonic value (‘liking’) and motivational salience (‘wanting’) of rewards. 12 When several rewards are available, MOR agonism increases and antagonism decreases preference specifically for the most valuable option. For instance, rats ate fewer palatable cookies but not less standard chow after MOR antagonism, 13 while MOR stimulation enhanced sexual ‘wanting’of only estrous, but not nonestrous, females. 14 We predicted that antagonism of the human opioid system would decrease, while MOR agonism would increase ‘liking’and ‘wanting’6 specifically for the evolutionarily most valuable option, namely attractive oppositesex faces.

In this double-blind, placebo-controlled cross-over study, 30 healthy males (see Figure 1 and Supplementary Information) viewed photographs of faces of varying attractiveness levels. In each session, participants received a μ-opioid receptor agonist (morphine 10 mg), a nonselective opioid receptor antagonist (naltrexone 50mg) or placebo, and performed one ‘liking’and one ‘wanting’task (see Parsons et al. 15). In the ‘liking’task, participants