Reviewed: Sellaro R, van Dijk WW, Paccani CR, Hommel B, Colzato LS. A question of scent: lavender aroma promotes interpersonal trust. Front Psychol. January 2015;5:1486. doi:10.3389/fpsyg.2014.01486.

Previous studies have indicated that the scent of lavender (Lavandula spp., Lamiaceae) has calming effects and induces a more inclusive cognitive-control mode,^1,2 whereas the scent of peppermint (Mentha × piperita, Lamiaceae) has stimulating effects and induces a more exclusive cognitive-control mode. (Cognitive control “refers to processes that allow information processing and behavior to vary adaptively from moment to moment depending on current goals, rather than remaining rigid and inflexible.”³) Other research suggests that interpersonal trust can be enhanced in a more inclusive cognitive-control state.4 Based on these observations, the aim of this study was to test if aromas are linked to interpersonal trust in healthy young adults exposed to the scent of lavender or peppermint while playing a behavioral trust game.

A total of 90 subjects aged 18-24 years, consisting of 68 women and 22 men, participated in the study conducted at the Leiden Institute for Brain and Cognition at Leiden University in the Netherlands. Subjects were excluded if they used drugs or had a psychiatric disorder, as assessed by the Mini International Neuropsychiatric Interview (MINI).

Participants were randomly distributed into three groups (30 in each group) that included the following: those who played the trust game in a lavender-scented room, a peppermint-scented room, or in a non-scented room (control group). Subjects were told that they were in a decision-making study, but were not informed about the study’s hypothesis or that there would be aromas in the room.

The aromas were produced by a candle diffuser containing four drops of lavender or peppermint essential oil (De TuinenTM Aromatherapie; Beverwijk, the Netherlands) in 30 mL of water. The diffusers were out of sight and lighted 20 minutes before the testing session to ensure uniform diffusion. The experiment was conducted with unacquainted same-sex dyads (two-person groups). Subjects rated their affective state with an affect grid (a 9 × 9 pleasure × arousal grid) before and after the trust game.

The trust game lasted about three minutes and involved having two subjects believe that they were either playing the role of “trustor” or “trustee.” In reality, all subjects were trustors. The participants were provided 5 euros and informed that any money would be tripled if given to the trustee. Once the trustor transferred the money, subjects were told that the trustee would decide how to distribute the money between both of them. Ultimately, this game was used to evaluate how much the subjects trusted one another with the money.

Overall, the researchers found that the trust score (defined as the amount of money transferred to the trustee in each experimental group) was impacted significantly by the aroma in the room (P = 0.03). Specifically, subjects in the lavender-scented room had a higher mean trust score (390 ± 125.5) than those in the peppermint-scented (323 ± 115.0; P = 0.03) or non-scented (320 ± 104.5; P = 0.02) rooms. No significant differences were found between the peppermint group and the control group (P = 0.90).

Based on the affect grid results, pleasure levels were found to be similar across groups and time. In contrast, multiple-comparison analysis indicated a significant group effect (P = 0.005) for arousal levels. Mean arousal scores were significantly lower in the lavender group (−0.07 ± 1.3) compared to the peppermint (0.90 ± 1.3; P = 0.008) and control (1.20 ± 1.3; P < 0.001) groups. The lack of correlation between the amount of money transferred and the levels of arousal or pleasure suggests that the subjects were not conscious of these feelings when transferring money.

The authors report that this is the first study to demonstrate that exposure to an aroma can affect interpersonal trust. However, the authors point out that this study could be improved by including physiological measurements and blind experimenters. Interestingly, only the lavender group had significantly different (higher) trust scores compared to the control group, which is consistent with a previous study that found similar effects when evaluating subjects with inclusive cognitive-control states.⁵ The lack of effect in the peppermint group to reduce interpersonal trust, as was hypothesized, should be explored in future studies that include a range of aromas associated with inclusive and exclusive cognitive-control states. No mention was made by the authors that previous studies^6,7 of essential oil effects differed between males and females.

—Laura M. Bystrom, PhD

References

1. Diego M, Jones NA, Field T, et al. Aromatherapy positively affects mood, EEG patterns of alertness and math computations. Int J Neurosci. 1998;96:217-224.
2. Field T, Diego M, Hernandez-Reif M, et al. Lavender fragrance cleansing gel effects on relaxation. Int J Neurosci. 2005;115:207-222.
3. Translational cognitive and affective neuroscience. Cognitive control. Carter Lab website. Available at: http://carterlab.ucdavis.edu/research/control.php. Accessed October 20, 2015.
4. Johnson AJ. Cognitive facilitation following intentional odor exposure. Sensors. 2011;11:5469-5488.
5. Sellaro R, Hommel B, de Kwaadsteniet EW, van de Groep S, Colzato LS. Increasing interpersonal trust through divergent thinking. Front Psychol. 2014;5:561.
6. Gilbert AN, Knasko SC, Sabini J. Sex differences in task performance associated with attention to ambient odor. Arch Environ Health. 1997;57:195-199.
7. Lehrner J, Eckersberger C, Walla P, Pötsch G, Deecke L. Ambient odor of orange in a dental office reduces anxiety and improves mood in female patients. Physiol Behav. 2000;71(1-2):1-15.