We found that MDMA (1.5mg/kg only) altered a behavioral indicator of social cognition. Specifically, it robustly reduced recognition of fearful faces, without changing recognition of other emotions from facial or vocal cues. Although previous studies have confirmed that the drug induces subjective feelings related to sociability and empathy, this is the first published demonstration of an overt behavioral effect of MDMA in humans.
The pattern of findings in our study may be more consistent with increased social approach behavior (i.e. sociability) than increased empathy. Anecdotal reports indicate that ecstasy increases interpersonal connection [3
], which may suggest increased sensitivity to emotions in others. In the present study, MDMA produced self-reports of loving feelings and friendliness, but it decreased
participants’ accuracy in identifying fear in others. A decreased ability to identify negative emotions, particularly threat-related signals such as fear, may facilitate social approach behavior [12
]. Thus, MDMA may facilitate social interactions because it reduces the impact of others’ negative emotions, rather than by enhancing recognition of, and sensitivity to, others’ emotions.
Unexpectedly, some subjective effects measured were increased by methamphetamine as well as MDMA. Indeed, only methamphetamine significantly increased self-rated sociability. Moreover, whereas MDMA (1.5mg/kg) significantly increased loving feelings and friendliness compared to placebo, these ratings were not significantly different from ratings on methamphetamine. These similarities were unexpected based on the different pharmacological profile and anecdotal reports of the effects of amphetamine and MDMA [36
]. It is possible that the expectation of receiving MDMA influenced subjects’ reports of empathogenic feelings. Although expectancies were minimized using double-blind conditions, and a range of possible drugs to be administered, it was difficult to completely control expectancies. Indeed, 6 participants of 21 guessed that they had consumed ecstasy when they had received methamphetamine. However, follow-up analyses revealed that participants’ guesses about the drug received were not significantly related to outcome measures, suggesting that expectancies play a minor role in these results. A more likely alternative is that methamphetamine may have a more prosocial profile than previously thought. Psychostimulants have been shown to enhance the rewarding value of social interactions in rodents [38
] and humans [39
], and methamphetamine is associated with social, and particularly sexual, enhancement in some subcultures [40
]. Thus, the prosocial effects of MDMA may be less selective than believed.
The effect of MDMA on social cognition was limited to identification of emotions from facial expressions, with no apparent effect on recognition of affective cues from the eye region or from voices. Thus, it appears that performance alterations arising from MDMA are specific to processing of whole-face visual affective cues. However, whereas the facial task required participants to identify emotions based on subtle, briefly presented pictures, there were no time limits on responding to items in the Eyes task. Moreover, whereas the vocal stimuli presented contained two levels of emotion intensity (high versus low), the facial task included 10 levels (from neutral to 100% intensity, in 10% increments). Thus, it may be that MDMA affects identification of subtle emotional cues, rather than having modality-specific effects. Future research requiring affect recognition from subtle, briefly presented vocal and eye-region stimuli could clarify this issue.
Some limitations should be noted. First, as noted above, the behavioral tasks we used may not have been sensitive to all of the unusual social effects attributed to MDMA. For example, the static photos of facial affect we employed may be less ecologically valid than dynamic stimuli [e.g. see 41
]. At the time of study initiation, no suitable dynamic stimuli were available. Second, we are not aware of tasks that include both social and non-social stimuli, making it difficult to determine the specificity of these findings to social stimuli. A third possible limitation was that we adjusted the dose to the body weight for MDMA but not for methamphetamine. This is unlikely to be a major factor because participants were within a narrow weight range (BMI 18.5 – 30), and post-hoc analysis indicated that only one measure of methamphetamine’s effects (playfulness) correlated with body weight (r
= − 0.45, p
= 0.04). A final limitation is that, for ethical reasons, all participants had prior experience with ecstasy or MDMA, so the findings may not generalize to MDMA-naive individuals [see 42
]. However, we intentionally recruited candidates with limited previous exposure to minimize possible effects of prior use.
There are a number of directions for future enquiry. In rodents, MDMA increases prosocial behavior [36
] and this effect is attenuated by an oxytocin receptor antagonist [2
]. Oxytocin is a neuropeptide known to be a critical modulator of social behavior in mammals [44
]. In humans, MDMA increases plasma oxytocin levels [13
], and plasma oxytocin levels also vary positively with enhanced subjective sociability after MDMA [13
]. This suggests that a fruitful area of future research will be to examine the role of oxytocin in the social cognitive, affective and behavioral effects of MDMA in humans. Second, there is a need for studies of the cognitive mechanisms underlying alterations to social cognition. For example, we found that MDMA (1.5mg/kg) increased the number of non-neutral faces erroneously identified as neutral, apparently reducing the capacity to detect subtle emotional signals. This phenomenon, which may underlie the reduced fear identification demonstrated in this study, requires further investigation. Finally, it is possible that certain social predispositions, such as high or low levels of trait sociability, affect the degree to which MDMA alters social processing in humans, potentially contributing to individual preferences for MDMA.
The present findings have implications for both recreational and therapeutic MDMA use. Recreational ecstasy users may benefit from knowledge that ecstasy, while increasing feelings of interpersonal connection, can subtly impair interpersonal competence. For instance, compromised social cognition may increase social risk-taking while under the influence of the drug. In addition, considering that social expectancies predict use of other drugs such as alcohol [47
], modifying users’ expectations about positive social effects of MDMA may alter ecstasy-use behavior [see 48
]. Regarding therapeutic use of MDMA, ongoing research on MDMA-assisted therapy should investigate possible mechanisms of any therapeutic effects [1
]. A recent placebo-controlled pilot study has indicated that MDMA-assisted psychotherapy may be useful in reducing symptoms in patients with treatment-resistant post-traumatic stress disorder (PTSD). Although mechanisms of this apparent effect remain unclear, reductions in the intensity of fear perception, including but not limited to those reported here, might facilitate engagement with traumatic material during therapy [49
]. Moreover, reduced sensitivity to subtle signs of negative emotions in others (e.g., the therapist) may enable a patient to express thoughts or feelings that were previously inhibited because of perceived negative responses in the listener. In addition to further controlled research to investigate the effectiveness of MDMA in psychotherapy, an understanding of the socioemotional cognitive mechanisms underlying such effects will help clinical researchers design treatments that optimize the drug’s therapeutic potential.