Prosocial effects of MDMA: A measure of generosity

MDMA (3,4-methylenedioxymethamphetamine) is reported to produce prosocial or empathogenic effects such as increased sociability and interpersonal closeness, but the cognitive and behavioural mechanisms underlying these effects are not well characterised. Earlier experimental work has been limited by a lack of objective laboratory tasks tailored to measure specific components of prosociality, such as generosity. Prior studies have shown that oxytocin can increase generosity in monetary allocation tasks and that MDMA raises plasma oxytocin, suggesting a possible mechanistic link; a recreational dose of MDMA has also been reported to increase prosocial choices in a resource allocation paradigm. This paper uses a novel Welfare Trade-Off Task (WTT), adapted from social discounting and delay-discounting approaches, to quantify generosity as the willingness to forgo personal monetary gain to benefit another person. Kirkpatrick and colleagues report two studies. Study 1 describes WTT performance in 361 healthy young adults and tests associations with personality (NEO-FFI) and socioeconomic status. Study 2 is a within-subject, double-blind trial in 32 MDMA-experienced volunteers that tested acute effects of two MDMA doses (0.5 and 1.0 mg/kg) versus placebo on WTT generosity toward a close friend and a stranger. The investigators predicted dose-dependent increases in generosity regardless of social proximity.

Study 1 recruited healthy adults aged 18–35 (N = 361; 57% female) as part of a larger genetic study; inclusion required at least a high school education and fluency in English, and participants were restricted to Caucasian ethnicity for that larger project. Screening excluded current psychiatric disorders and medication use. Participants completed a single laboratory visit after abstaining from alcohol, marijuana and other recreational drugs for specified intervals; abstinence was verified by breath and urine tests. Personality was assessed with the NEO-FFI (five traits), and household income was reported on a 21-point scale then grouped into four roughly equal categories. The primary behavioural measure was the computerized Welfare Trade-Off Task (WTT): for a named close friend and a named acquaintance participants made a series of hypothetical choices between receiving money for themselves or giving a larger amount to the other person. Anchor values for the recipient were fixed ($19–$75 across six series), while the self-payoff varied to produce a set of Welfare Trade-Off Ratios (WTRs). The switch point where a participant changed from preferring self to preferring the other person is taken as the WTR; higher WTRs indicate greater generosity. Analysis in Study 1 comprised a paired t-test comparing Friend versus Acquaintance WTRs and two multiple regression models (forced entry) predicting WTR for Friend and for Acquaintance from eight predictors (sex, age, income group, and the five NEO-FFI traits). Stepwise follow-up regressions were used to identify more parsimonious models and verify significant predictors; significance was set at p < 0.05. Study 2 enrolled MDMA-experienced healthy adults aged 18–30 (N = 32; 9 female, 23 male) who had used MDMA between 4 and 80 times. Medical and psychiatric screening (including ECG) excluded individuals with conditions that increased study risk. The design was within-subjects and double-blind: each participant completed three outpatient sessions separated by at least 5 days and received placebo, 0.5 mg/kg MDMA and 1.0 mg/kg MDMA in randomized order; average absolute doses were ~36 mg and ~72 mg given participants’ average weight. Participants provided biological samples to confirm abstinence before each session; capsules contained MDMA powder or lactose placebo. The WTT was completed at approximately 90 minutes after dosing (11:00), and other physiological and subjective measures were collected at multiple time points. For Study 2 the WTT probed generosity toward a named close friend and a designated stranger; post-hoc checks found acquaintance and stranger switch points were comparable across studies. Repeated measures ANOVA tested Drug Dose (placebo, 0.5, 1.0 mg/kg) and Person (Friend, Stranger) as within-subject factors with Participant Sex as a between-subjects factor. Significant main effects or interactions were followed by one-tailed t-tests comparing doses; alpha was 0.05.

Study 1: On the WTT participants were more generous to a close friend than to an acquaintance (mean WTRs 0.65 ± 0.02 versus 0.41 ± 0.02; t(359) = 17.4; p < 0.001). The full regression model predicting Friend WTR was not significant (F[8,360] = 1.5, p = 0.17, R2 = 0.032), but a parsimonious model retained Agreeableness alone (F[1,360] = 8.7, p = 0.003, R2 = 0.024). In that model each one-point increase on the Agreeableness scale was associated with a 0.006 point increase in the Friend switch point (95% CI 0.000–0.012; t(360) = 2.1; p < 0.05). Put differently, using the paper's illustrative conversion, for a friend who could receive $100 a participant at the lowest possible Agreeableness would trade off about $41 whereas a participant at the highest Agreeableness would trade off about $70. For the Acquaintance, the full model was significant (F[8,360] = 2.6, p = 0.009, R2 = 0.056). The best-fitting model included Agreeableness and Income group (F[2,360] = 8.5, p < 0.001, R2 = 0.046). Agreeableness increased Acquaintance switch points by 0.008 per point (95% CI 0.003–0.013; t(360) = 3.0; p < 0.005). Household income was negatively associated with generosity to an Acquaintance (B = -0.038; 95% CI -0.067 to -0.010; t(360) = 2.7; p < 0.01), indicating lower-income participants were more generous to acquaintances (example conversion: lowest income group willing to trade off about $11 more than highest income group when the acquaintance could receive $100). No other demographic or personality traits (including Extraversion) significantly predicted WTRs. Study 2: Across drug conditions participants remained more generous to Friends than Strangers (main effect of Person: F(1,30) = 22.2; p < 0.001). There was a Dose × Person interaction (F(2,60) = 3.5; p < 0.05). Specifically, the higher MDMA dose (1.0 mg/kg) increased WTR switch points toward Friends compared with placebo (comparison between 1.0 mg/kg and placebo: t(31) = 1.8; p < 0.05). The authors illustrate this as a change from trading off about $57 under placebo to about $72 under 1.0 mg/kg MDMA (an increase of roughly $15 for a hypothetical $100 recipient). When the other person was a Stranger, the lower dose (0.5 mg/kg) produced a modest increase in WTRs in women but not in men, as indicated by a Dose × Sex quadratic interaction (F(1,30) = 4.8; p < 0.05). For women the mean difference between 0.5 mg/kg MDMA and placebo on Stranger WTR was 0.25 ± 0.11 (95% CI 0.03–0.48; p < 0.05), whereas for men the mean difference was 0.04 ± 0.07 (95% CI -0.10–0.18; p = 0.56). No other sex differences in drug response reached significance.

Kirkpatrick and colleagues interpret the findings as supporting the WTT as a useful laboratory measure of a facet of prosocial behaviour — generosity — that is sensitive to social proximity and to individual differences. The key descriptive results were that participants were more generous to close friends than to acquaintances or strangers, that higher trait Agreeableness predicted greater generosity to both friends and acquaintances, and that lower household income was associated with greater generosity to acquaintances but not to friends. These patterns align with prior observations that agreeableness relates to prosocial dispositions and that socioeconomic status can inversely relate to generosity in some contexts. Regarding pharmacological effects, the investigators report that MDMA increased generosity in a dose- and relationship-dependent manner: the 1.0 mg/kg dose increased generosity toward a close friend but not a stranger, whereas the lower dose produced a small increase in generosity to strangers primarily in women. The authors relate these findings to prior work on oxytocin and prosocial behaviour, noting that MDMA elevates plasma oxytocin and that intranasal oxytocin has context-dependent effects (for example, preferentially enhancing in-group trust). They suggest the observed dependence on social proximity may be consistent with oxytocin-related mechanisms but acknowledge that serotonin or other non-specific effects may also contribute. The paper notes several limitations acknowledged by the authors. Study 1 used a relatively homogeneous sample (young, Caucasian, screened for psychiatric symptoms), limiting generalisability. Study 2 had a small sample size, which constrains confidence in subgroup findings such as the sex-specific effect for strangers. The mechanism linking MDMA to increased generosity was not directly tested (for example, oxytocin levels were not measured in these data), and the effects of dose order or longer-term persistence of prosocial changes were not established. The authors recommend that future research examine biological mediators (e.g. oxytocin), include larger and more diverse samples, and compare MDMA's effects with other psychoactive drugs. In closing, the investigators conclude that the WTT captures an interpretable component of prosocial behaviour and that MDMA can increase generosity in a manner shaped by social relationship; they suggest the task will be useful for further studies of prosocial effects of MDMA and other substances, and note that such prosocial effects may contribute to MDMA's recreational appeal and abuse potential.