“Our results offer the first evidence that pharmacological manipulation of the human MOR system affects both aesthetic evaluation of and motivation for viewing opposite-sex faces. In line with findings from rodent literature,13,14 the effects of the MOR manipulations were strongest for the most valuable stimuli, that is, the most beautiful women. Morphine increased and naltrexone decreased men’s ‘liking’ of these faces.
We also observed an increase in ‘wanting’ behavior after morphine relative to naltrexone treatment, indicating that manipulation of the opioid system affected participants’ motivation to expend effort.
Specifically, activation of the opioid reward system with morphine not only increased ‘wanting’ key-press behavior to keep viewing the beautiful faces but also increased motivation to avoid viewing the least attractive faces.
The two components of reward, hedonic evaluation (´liking´) and motivational salience (´wanting´) were previously shown to partially dissociate when men viewed female faces of varying attractiveness levels,6 and when males and females viewed images of infants.15 The current study revealed an opioid-related increase in motivation to avoid the least attractive female faces, which was not mirrored by changes in reported attractiveness of these faces. For the most beautiful faces, however, the MOR manipulations affected ‘liking’ and ‘wanting’ similarly and in the expected directions.11
Together, these findings suggest that the human opioid system may mediate social motivation by enhancing the salience and reward appraisal of the most valuable stimuli, while inhibiting ‘wanting’ of less valuable social cues.