The level of dopamine receptors in a person’s brain can influence whether he or she likes or dislikes the effects of a drug. Because dopamine is involved in the rewarding effects of drugs of abuse, it was hypothesized that normal individual variation in dopamine receptors could influence a person’s response to drug exposure. In this study, human subjects were given the stimulant methylphenidate (Ritalin), and their brain’s were imaged using PET. Those subjects who had high levels of dopamine receptors found the experience unpleasant; while those with lower levels of dopamine found it pleasant. This suggests that individual differences in a marker of dopamine function can influence an individual’s susceptibility to addiction.