We focused our analysis on trials where participants made choices between virtues and vices. Table 3 presents the proportion of healthy choices (i.e., choices for the virtue over the vice) across the three conditions.

Table 3 Proportion of healthy choices across drug conditions Full size table

A generalised linear model (generalised estimating equations) was run with the proportion of healthy choices as the dependent variable, and Drug condition and Session as within-subjects factors. The test of model effects revealed that there was a significant main effect of Drug condition, Wald chi-square (2) = 7.95, p = .0188; and the parameter estimates revealed that, relative to placebo, citalopram was a significant positive predictor of healthy choice, B = .276, Wald (1) = 7.96, p = .0048. There was neither a significant main effect of Session, Wald (2) = 1.59, p = .4515, nor an interaction between Drug and Session, Wald (4) = 5.09, p = .2782; which shows that our design successfully avoided any order effects. In a pairwise, Bonferroni-corrected comparison between the drug conditions, the estimated marginal means for the effect of Drug revealed that on citalopram participants made a significantly higher proportion of healthy choices relative to placebo (p = .0048), while the proportion of healthy choices did not significantly differ between atomoxetine and placebo (p = .1531), or between atomoxetine and citalopram (p = .2575).

We also measured individual differences in eating behaviour using the Restrained Eating Scale (Herman & Polivy, 1975). This scale measures preoccupation with weight and eating behaviour, and participants with scores higher than 17 are classified as restrained eaters. In our sample, the mean score (+SD) was 9.1 (+3.6). One participant had a score of 18; and the results did not change when excluding this participant from the analysis—for example, similarly significant: main effect of drug condition, Wald chi-square (2) = 6.52, p = .0384; parameter estimate of the citalopram effect, B = .148, Wald (1) = 6.30, p = 0.0121; and estimated marginal means effect of citalopram relative to placebo, p = .0315.

We next investigated how the drugs affected health and taste ratings of virtues and vices using a repeated-measures general linear model, with rating as the dependent variable and Drug (citalopram, atomoxetine, placebo), Attribute (taste and health), and Food (virtue vs. vice) as within-subjects factors. The results revealed neither a significant main effect of Drug, F(2, 52) = 0.21, p = .810, nor an interaction between Drug and Attribute, F(2, 52) = 0.62, p = .544, or between Drug and Food, F(2, 52) = 0.28, p = .761, which suggests that the drugs did not impact participants’ ratings of the healthiness or tastiness of the food items. Crucially, there was a significant Food × Attribute interaction, F(1, 26) = 237.68, p < .001, where health ratings were higher for virtues than vices (M virtue = 443.31, SD = 42.92; M vice = 197.81, SD = 49.97), while taste ratings were not different for vices and virtues (M virtue = 380.71, SD = 53.49; M vice = 377.72, SD = 74.84). There was also a significant effect of type of Food (virtues, vices), whereby virtue foods were rated higher than vice food overall across both health and taste ratings, F(1, 26) = 146.09, p < .001; but this effect (across all drug conditions) was driven by the difference in the health attribute rather than the taste attribute. There was also a main effect of Attribute, F(1, 26) = 63.37, p < .001, where taste ratings were higher than health ratings overall. In summary, those results confirm that our selected stimuli were appropriately distinguished on the basis of their healthiness.

We assessed the impact of health and taste dimensions on choice by computing for each attribute the difference in ratings between the chosen and unchosen options (see Table 4). The repeated-measures analysis used Drug and Attribute (health vs. taste) as a within-subject factors, with the difference in ratings between the chosen and unchosen options as the dependent variable. There was a significant main effect of Attribute, F(1, 26) = 6.193, p < .05, where the chosen–unchosen difference for taste ratings (M = 68.29, SE = 6.83) was greater than the chosen–unchosen difference for health ratings (M = 24.15, SE = 15.42), suggesting that, overall, our participants weighted taste more strongly than health in their choices. However, there was also a significant interaction between Drug and Attribute, F(2, 52) = 6.577, p < .01. As Table 4 shows, there was little variation across the three conditions in terms of the taste (chosen–unchosen) rating differences, while the health (chosen–unchosen) ratings showed large disparities between the conditions. Citalopram showed the largest difference between ratings in the health dimension. This pattern implies that on citalopram, participants more frequently selected the option that had the higher health rating (i.e., more often the chosen food had a higher health rating than the rejected food).

Table 4 Difference between chosen and unchosen foods across healthiness and liking attributes for each drug condition Full size table

One possible alternative explanation for our findings is that citalopram did not affect decisions by modulating goal values per se, but rather made participants less hungry, which in turn made them less likely to select the vice. To test this, we first examined whether citalopram and atomoxetine affected hunger ratings. Baseline hunger levels were moderate across all conditions (M citalopram = 4.59, SD = 1.42; M atomoxetine = 4.82, SD = 1.21; M placebo = 5.46, SD = 0.97). A repeated-measures general linear model with hunger as the dependent variable and Drug (citalopram, atomoxetine, placebo) as a within-subject factor revealed a significant main effect of Drug, F(2, 25) = 6.56, p = .005. Tests of within-subjects linear contrasts revealed that both citalopram, F(1, 26) = 12.43, p = .002, and atomoxetine, F(1, 26) = 6.57, p = .017, reduced the ratings of hunger relative to placebo. The same pattern was exposed by Bonferroni-corrected pairwise comparisons between the marginal means for citalopram and placebo (p = .005), atomoxetine and placebo (p = .050), but not citalopram and atomoxetine (p = 1.000). Decreased appetite is a clinically well-known side effect of atomoxetine (Michelson et al., 2003), but not of citalopram (Khawam, Laurencic, & Malone, 2006). Furthermore, given that only citalopram affects choice, we tested whether individual differences in the citalopram effect on hunger correlated with the citalopram effect on choice (i.e. those subjects who showed stronger citalopram effects on hunger also showed the stronger citalopram effects on choice). This correlation was indeed negative and significant, r(27) = -.42, p = .029, which warranted a mediation analysis (Judd, Kenny, & McClelland, 2001) to test whether changes in hunger caused by citalopram mediated drug-induced changes in self-control. In the mediation analysis, the direct effect of citalopram on choice was again significant, t(26) = 2.51, p = .0186. The indirect (mediation) effect of citalopram on choice through hunger was only marginally significant (Z = 1.75, p = .081). Therefore, citalopram’s effects on food choice could partially be attributed to changes in appetite.

Finally, citalopram-induced changes in dietary choice could be caused by nonspecific side effects, such as nausea, of citalopram (Khawam et al., 2006). In our study, we also asked the participants to rate their experience of eight ‘mood’ states in every condition. We ran a multivariate ANOVA (GLM) with Mood ratings as the dependent variable and Drug as the within-subjects measure. All mood ratings were insignificant, apart from “nauseous”: positive affect: F(2,81) = 1.572, p > .05; negative affect: F(2, 81) = 1.290, p > .05; drowsy: F(2, 81) = .570, p > .05; hostile: F(2, 81) = .390, p > .05; irritable: F(2, 81) = .195, p > .05; energetic: F(2, 81) = .593, p > .05; attentive: F(2, 81) = .967, p > .05; nauseous: F(2, 81) = 6.079, p = .003. Planned comparisons showed that nausea was higher in the atomoxetine (M = 65.61, SD = 98.34) than in the citalopram (M = 31.46, SD = 86.74) and the placebo (M = -11.11, SD = 87.39) groups, respectively. We ran a mediation analysis of the citalopram versus placebo effect on choice, with nausea as the mediator, but the mediation effect was not significant (Z = 0.99, p = .324), while the total effect of citalopram on choice was significant, t(26) = 2.51, p = .0186.