Sour promotes risk-taking behaviour

In the first experiment (see Fig. 1), we collected data from seventy participants in the UK (46 females, mean age 25.0 ± 6.4). Each participant performed two blocks of the BART task after ingesting one of the five basic tastes or mineral water (neutral solution). Analysing the unexploded and uncontaminated balloons, our results show that (i) ‘sour’ promoted high risk-taking behaviour, (ii) ‘sweet’ and ‘umami’ supported low risk-taking behaviour; and (iii) ‘bitter’ and ‘salty’ triggered neither risky nor safe behaviour. Fig. 2(a) provides an overview of the average adjusted number of pumps for unexploded and uncontaminated balloons as key indicator for participants’ risk-taking behaviour. On average, participants who ingested sour pumped the balloons 39.36 times for unexploded balloons. This is significantly higher than with any other taste (39.08% more than sweet, 20.50% more than bitter, 16.03% more than salty, and 40.29% more than umami). In other words, participants aimed to maximize their cash reward by taking greater risks with the sour taste (see Fig. 2(b) for the accumulated money (represented in points, vertical axis) over time for each stimulus in the first experiment).

Figure 2 Experiment 1 results with 70 UK participants. (a) The average adjusted number of pumps for unexploded and uncontaminated balloons indicating risk-taking behaviour across all 5 basic taste groups (**p < 0.01, ***p < 0.001). (b) Accumulated cash reward (in points, vertical axis) earned by participants in each stimulus group (uncontaminated trials) in a time-course of a block of 30 trials (horizontal axis). (c) The average inter-click time between pump-up actions for uncontaminated (both exploded and unexploded) balloons across all five stimulus groups. (d) Average inter-click time (in seconds) across all trials, organized by stimulus group and by unexploded and exploded balloons. Pairwise comparisons found significant differences in all stimuli and between exploded and unexploded balloons (*p < 0.05, **p < 0.01, ***p < 0.001). Bars represent standard error of the mean (SE). Full size image

Analysing the participants pumping behaviour in more detail, our results showed that the sweet, followed by the sour taste, made participants hesitate the most (decide slower). Figure 2(c) shows that the inter-click time for pumping up balloons in the sour group was significantly higher (M 0.38 seconds SE 0.01) compared to the salty (M 0.30 seconds SE 0.02), bitter (M 0.27 seconds SE 0.01), and umami (M 0.23 seconds SE 0.01) groups, but similar to the sweet group (M 0.42 seconds SE 0.03). When further comparing pairs of exploded versus unexploded balloons in terms of their inter-click times, we found significant differences between the taste groups. Participants in the sour and salty group exhibited significantly (p < 0.001) greater inter-click times on pumping up exploded balloons than on unexploded balloons. Figure 2(d) shows that this behaviour was reversed for bitter, sweet and umami, as well as for the neutral stimulus, where participants exhibited greater inter-click times on the unexploded balloons. In summary, from this first exploratory experiment, we learned that the effect of the five basic tastes on risk-taking behaviour is divided into three clusters, where sour promotes the highest and sweet and umami the lowest risk-taking, while no clear picture was obtained for bitter and salty.

Sour promotes risk-taking across two countries

To further understand the effect of taste on risk-taking and account for known cultural differences in taste perception, in particular for the umami taste9,23,24, we repeated the same experiment with an identical procedure and sample size (N = 71) in Vietnam (see Fig. 3). Vietnam has the 3rd largest mono-sodium glutamate (MSG) consumption28 (MSG is a compound used to present the umami taste). Despite being rich in protein and nutritious, umami in its pure form is often perceived as unpalatable in Western countries. Due to the higher consumption of MSG-rich food in Asian countries28, a different perception and reaction could be expected. We hypothesized that the low risk-taking effect of umami would be confirmed, but that the temporal pattern would change from the fastest inter-click time in the UK sample, to a slower inter-click time, and consequently closer to sweet. This was based on the assumption that the perceived pleasantness of the taste changes across the two countries and modulate the effect in the BART task.

Figure 3 Experiment 2 results with 71 Vietnamese (VN) participants. (a) The average adjusted number of pumps for unexploded and uncontaminated balloons) indicating risk-taking behaviour across all 5 basic taste groups (**p < 0.01, ***p < 0.001). (b) Accumulated cash reward (in points, vertical axis) earned by participants in each stimulus group (uncontaminated trials) in a time-course of a block of 30 trials (horizontal axis). (c) The average inter-click time between pump-up actions for uncontaminated (both exploded and unexploded) balloons across all five stimulus groups. (d) Average inter-click time (in seconds) across all trials, divided by stimulus group and by unexploded and exploded balloons. Pairwise comparisons found significant differences in all stimuli and between exploded and unexploded balloons (**p < 0.01, ***p < 0.001). Bars represent standard error of the mean (SE). Full size image

The results from the second experiment confirmed the sour taste as the unique taste quality promoting the highest risk-taking behaviour. Figure 3(a) shows an overview on the average adjusted number of pumps for unexploded and uncontaminated balloons and (b) accumulated monetary reward (represented in points). While, those results confirmed sour in promoting the highest risk-taking behaviour (M 50.88 ± 0.99), no confirmation was obtained for the other four tastes, which can be arranged into an increasing order (from lowest to higher risk-taking behaviour): salty (M 28.46 ± 1.03), bitter (M 36.00 ± 0.98), umami (M 38.37 ± 1.33), and sweet (M 41.47 ± 1.25). Surprisingly, in contrast to the UK sample, both sweet and umami changed to promote riskier decisions.

We explored the temporal patterns, analysing the inter-click time between pump-up actions for the Vietnamese participants. Figure 3(c) shows that sour exhibited, to our surprise, the fastest inter-click times (M 0.37 seconds SE 0.01) in contrast to the results in the first experiment, were the inter-click time for sour was - after sweet - the slowest amongst the tastes. Interestingly, however, despite this different positioning of sour in the range of the other tastes, there was no significant difference between its inter-click times for the two experiments (i.e., 0.38 versus 0.37 seconds in the UK and Vietnam, respectively). That means, that sour not only remains the unique taste quality to promote risk-taking but the sour effect on users’ performance (inter-click time) is stable across two countries (different cultural contexts).

When analysing the exploded and unexploded pumping up actions separately, the results from the second experiment show that participants exhibited significantly different inter-click times for balloons that were going to be cashed out (unexploded balloons) than for those that were going to explode. This was true for all tastes but sweet, where no significant difference was found. With respect to sour, we can see a difference between the UK and Vietnamese sample with respect to sour to the extent that the UK participants exhibited significantly greater inter-click times on the exploded balloons (Fig. 2(d)), while the Vietnamese participants exhibited greater inter-click times on the unexploded balloons (Fig. 3(d)). This distinction could be explained through differences in the strategies applied by the participants in the two samples (please note that the UK sample was more diverse with respect to the participants background compared to the Vietnamese sample, which was fully recruited from the School of Biotechnology and Food Technology, Hanoi University of Science and Technology).

Taken together, the results from both the first and second experiment provide compelling evidence for sour in promoting riskier behaviour across two countries and risk-taking personalities (in both experiments we controlled and balanced low and high risk-takers). To account for individual differences in strategies when completing the BART task, we designed a third experiment controlling for the differences in the individual participants’ strategies and decision making (i.e., style of thinking).

Sour promotes risk-taking in analytic and intuitive thinkers

In the third experiment (see Fig. 4), we explicitly informed participants about the average explosion point in the BART tasks (64 pumps), hence reducing the level of uncertainty in their decision making. Each participant performed two blocks of the BART task and was presented either with a sour stimulus or mineral water, in a counterbalanced order. We expected participants to align their strategies in the pumping up actions, reflected in an increase in the risk-taking behaviour in both conditions. Additionally, we controlled for individual differences in style of thinking (i.e., intuitive versus analytic). We used the Cognitive Reflection Test (CRT)29 and marked participants who answered two or more from the three questions correctly as “analytic”, whereas participants who answered less than two questions correctly were marked as “intuitive”29. We hypothesised that participants applying an analytic approach would make riskier choices to get a higher reward in comparison to intuitive thinkers.

Figure 4 Experiment 3 (UK-Control) results and comparison to the results from the first (UK) and second (VN) experiments: (a) Average adjusted number of pumps for unexploded and uncontaminated balloons for each stimulus (neutral and sour) accounting for intuitive and analytic thinkers based on the Cognitive Reflection Test (CRT); (b) Comparison of the adjusted number of pumps for unexploded and uncontaminated balloons of sour in three experiments: 70 UK participants (experiment 1), 71 Vietnamese participants (VN - experiment 2), 16 analytic vs. 11 intuitive participants (UK-Control - experiment 3); (c) The same comparison with the inter-click times representing the hesitation level. Bars represent standard error of the mean (SE). Full size image

As before, sour again significantly promoted risk-taking behaviour compared to the neutral stimulus (p < 0.05). Figure 4(a) shows that the average adjusted number of pumps for unexploded and uncontaminated balloons was significantly higher for the sour group (M 43.76 SE 0.85) compared to the neutral stimulus (M 40.83 SE 1.13).

Participants in both sour and neutral conditions exhibited the same distinct risk-taking behaviour. Participants following an analytic style of thinking took significantly more risk (M 43.77 SE 1.05) compared to participants following an intuitive style of thinking (M 40.80 SE 0.93, p < 0.05). These results suggest that sour promoted riskier choices regardless of the style of thinking. It remains to be explored as to how the style of thinking is correlated with a persons’ risk-taking personality.

Moreover, when the uncertainty in the BART task was reduced (participants in the 3rd experiment - UK-Control were informed about the average explosion point), our results show that user’s performance significantly improved compared to the first (13% faster) and second experiment (11% faster) (Fig. 4(c)). These results indicate that participants in experiment 3 performed faster under reduced uncertainty, while sour consistently promoted riskier decisions across all three experiments (see Fig. 4(b) for comparison between the three experiments). Additionally, our analysis of the inter-click time showed that the inter-click time slowly decreased over time across all taste stimuli and in all three experiments (linear regression slope in Experiment 1 is: M -0.0062 SD 0.0012, in Experiment 2 is: M -0.0059 SD 0.0015, in Experiment 3 is: M -0.0037 SD 0.0003). This result is similar to previous work that analysed inter-click time in each trial of the BART task synchronized with the captured EEG signals (by analysing the amplitude of the P30030). In this prior work, it has been shown that the slow decrease was due to participants’ hesitation at the beginning of the BART task. As participants progressed, they established the reward structure of the task and were clicking faster30.