Introduction

Trust, a broad term that essentially describes a capacity to have strong confidence in someone or something, fosters collaboration and enables constructive response to crises [1]. However, the COVID-19 pandemic crisis appears to have weakened rather than fostered trust within societies [2]. This erosion of trust may, at least partially, have been driven by social isolation during pandemic lockdowns, considering the importance of the multiple factors affecting interpersonal trust. These range from social relationships [3] at a broader level to oxytocin at a molecular level–and were likely all impacted by pandemic lockdowns [4]. With state structures, systems of governance and scientific institutions (democracy) also at stake in Western Europe, strategies to promote trust and thus engagement in society are urgently needed. The rapid increase in antidemocratic movements that rely on or appeal to conspiracy theories and to mistrust in scientific and governmental institutions observed during the COVID-19 pandemic calls for urgent action [5].

Interpersonal trust reflects the willingness of individuals to accept vulnerability due to expectations that are placed on the other person’s behavior, essentially encapsulating individuals’ predisposition to trust one another [6]. Interpersonal trust is central to human behavior and social life. It is essential for successful interpersonal relationships, and is considered a key component of social capital [7]. Studies on interpersonal trust need to be framed within a larger context that takes into account the trustworthiness of actors and the relationships between them [8]. The integrative neuropsychoeconomic (NPE) model of interpersonal trust proposed by Krueger and Meyer-Lindenberg [9] describes the evolution of trust from calculus-based, to knowledge-based, to identification-based trust, calling for the inclusion of aspects beyond the economic context and extending to other real-world scenarios (family, institutions). The NPE model also postulates that individual differences in propensity to trust may be driven by variability in functional connectivity in large-scale dynamic networks of the brain [9]. Importantly, this embedded perspective on trust calls for (lab and field) studies to improve our understanding of the interaction of culture, nurture, and nature in shaping human trust (or lack thereof) in specific social, political, and cultural contexts.

Despite extensive literature on determinants of interpersonal trust [10], to our knowledge, putative correlates of trust including a wide range of personality traits and psychological measures, have not been studied during the COVID-19 pandemic in specific regions and especially in the D-A-CH region. We therefore conducted a cross-sectional study of adults in the D-A-CH region to identify potential targets for interventions aimed at strengthening interpersonal trust in societies during times of crises. Given prior literature [11], including our own [12], we hypothesized that optimism would be a particularly strong predictor of trust. We conclude that targeting optimism as well as complexity thinking, which we found to be positively correlated with interpersonal trust, can have positive effects on building trust in our societies and can thus reinforce the role of scientific institutions in fostering democratic societies.

Materials and methods

Variables

The survey assessed numerous sociodemographic variables, including those listed below:

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https://doi.org/10.1371/journal.pone.0286488.t001

Additionally, we assessed participant’s main job task, satisfaction with work and work-life balance. For the latter, we utilized the validated “Trierer Kurzskala zur Messung von Work-Life Balance” (TKS-WLB) [13], which comprised 5 statements ranked from 1 (totally disagree) to 6 (totally agree) and rendered a score from 5 to 30 for which a higher score indicated better work-life balance. We also assessed several health factors, including body mass index (BMI), whether participants ever had tried to lose weight, and if so, their approach (e.g. nutrition, or other strategies), frequency of physical activity (for at least 10 minutes raising heartbeat or respiratory rate), smoking status, chronic disease and depression history, the frequency of sleep-related difficulties such as initiating sleep and/or maintaining sleep and/or waking up earlier than desired, and whether these sleep difficulties had been present for more or less than 3 months.

The survey also assessed several personality characteristics. These included optimism, for which we used the validated Life-Orientation-Test revised (LOT-R), which invites responses to six items on a five‐point Likert scale (0–4), rendering a score from 0 to 24 for which higher scores indicate higher optimism [14]. To assess interpersonal trust, we utilized the validated “Kurzskala für interpersonales Vertrauen” (KUSIV3). This instrument features three items that are rated on a scale of 1–5 and averaged [11]. The additional personality traits of empathy and perspective taking were assessed with the validated “Fragebogen für Empathie und Perspektivenübernahme.” Briefly, empathy and perspective taking are assessed separately, each with nine statements rated on a six-point Likert scale (0–5), rendering a score from 0 to 45 for each trait (with higher scores indicating stronger demonstration of the traits) [15]. Further, we assessed the “Big Five” personality traits, e.g., conscientiousness, extroversion, agreeableness, openness and neuroticism using the validated Big-Five-Inventory-SOEP (BFI-S) in which three statements per trait are rated on a seven-point Likert scale (1–7), rendering a score from 3 to 21 for each trait, with higher scores suggesting higher degrees of the corresponding personality traits [16]. Lastly, we assessed whether participants considered themselves to be a “No, but” or a “Yes, and” type in conversations. In improvisation, such as in improvisational theater, the “Yes, and” type is considered key for an ability to build spontaneously on previous ideas; it is also considered essential for effective collaboration [17]. We assessed this variable because we considered that a personality type associated with teamwork might also be related to interpersonal trust.

Additionally, several variables related to the COVID-19 pandemic were assessed, including history of COVID-19 infection (positive test), approval of the COVID-19 measures implemented by the government and vaccination against COVID-19. Finally, participants were also asked whether they agreed with the following two statements: 1. Complex real problems require collaboration between scientists and practitioners in problem solving. 2. I have heard of the Sustainable Development Goals (SDGs) and consider them to be important.

Conspiracy and complexity.

In order to assess conspiratorial and complex thinking we devised a set of additional questions. Survey participants indicated on a Likert scale (disagree, rather disagree, rather agree, agree) their views regarding the positive effect of daily activities (such as eating less meat or walking/cycling more) on their health and the environment/climate. Further, they indicated how much they agreed or disagreed with statements that investigate conspiratorial or complex thinking (using similar questions as in the survey on conspiracy theories implemented by Roose J [18]. Conspiracy and complexity score were developed according to these questions after principal component analysis (PCA), as described in detail elsewhere (see S1 File)

Briefly, PCA was used to identify the questions to be included for the conspiracy and complexity scores. Initially, we examined the factorability of the 12 survey questions intended for the construction of conspiracy and complexity score. All 12 of those questions correlated at least 0.3 with at least one other item, suggesting reasonable factorability (see S1 File). Additionally, the Kaiser-Meyer-Olkin measure of sampling adequacy was 0.86, above the commonly recommended value of 0.6, and Bartlett’s test of sphericity was significant (χ2 = 13143.96, p < 0.05). Eigenvalues of the first two principal components were 4.199 and 2.126, respectively, and explained 35% and 18% of the variance, respectively. The third, fourth and fifth factors had eigenvalues close to 1 and explained 9%, 7%, and 6% of the variance, respectively. Solutions for three, four, five, and six factors were each examined using oblimin rotations of the factor loading matrix. The two-factor solution, which explained 53% of the variance, was preferred because of its previous theoretical support [19] and the leveling of eigenvalues on the screen plot after two factors (see S1 File). For the final stage, we conducted a PCA using varimax and oblimin rotations, with two factors explaining 53% of the variance. The oblimin rotation provided the best-defined factor structure. All factors in this analysis had primary loadings over 0.55. The factor loading matrix for this final solution is presented (see S1 File).

Internal consistency for each of the scales was examined using Cronbach’s alpha [20]. The alpha was moderate, at 0.69, for the complexity score, and high, at 0.87, for the conspiracy score. By eliminating the candidate questionnaire statement, “Pandemics like COVID-19 are connected to climate change.” from the complexity score, the Cronbach’s alpha could have increased to 0.74, but the question was purposefully retained because we found the question to be of high importance to gauge complex thinking.

Composite scores were created, based on the sum of the items that had factor loadings >0.55 on each principal component. Higher scores indicated more complex thinking or more belief in conspiracy theories. Neither score was normally distributed as shown by the Shapiro Wilk’s test (p<0.001) and examination of the histograms (see S1 File). The conspiracy score was positively skewed, while the complexity score displayed negative skewness. Descriptive statistics for the scores are presented in Table 1.

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Table 1. Sample characteristics (N = 3,065) across tertiles of interpersonal trust.

https://doi.org/10.1371/journal.pone.0286488.t002

Statistical methods

Descriptive statistics were used to summarize characteristics of the whole study population across tertiles of interpersonal trust. For categorical variables, frequencies were reported and Pearson’s chi-squared tests used to test for differences between countries. For continuous variables, median and interquartile range (IQR) were reported after Shapiro-Wilk tests indicated that the continuous variables were not normally distributed. We used k-sample equality-of-medians tests to compare findings across tertiles of interpersonal trust. We investigated different transformation approaches but did not find one that adequately normalized the continuous variables. Thus, we decided to categorize continuous variables into tertiles based on the total study sample.

For the univariable analyses, we used multinomial logistic regression models to calculate odds ratios (OR) and 95% confidence intervals (CI) to investigate factors associated with interpersonal trust. Ordered logistic regression was not applied because the parallel regression assumption was violated. Too few participants reported a gender other than man or woman (n = 2) to be analyzed separately, and thus those participants were excluded from all analyses, leaving a final sample of 3,065 participants.

The following factors were investigated in univariable analyses:

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https://doi.org/10.1371/journal.pone.0286488.t003

For the multivariable regression analyses, we initially included all variables that were associated (p<0.05) with trust in univariable analyses. We then utilized backward stepwise selection to eliminate variables with a p-value ≥ 0.05 to arrive at our final models. We followed this approach for the whole sample and for each country and gender separately. Variance inflation factors (VIF) indicated no multicollinearity in the multivariable models (VIF<4). We used missing indicators to represent missing data in the models. A two-sided p-value less than 0.05 was considered statistically significant.

Transparency and promotion.

We describe our sampling plan, all data exclusions, all manipulations, and all measures in the study, and we adhered to the Journal of Applied Psychology methodological checklist. All data, analysis code, and research materials are available upon reasonable request. This study’s design and its analysis were not preregistered. All data analyses were performed using STATA (version 14.1, 2015, StataCorp LP).

Results

Correlates of trust

Among all 3,065 participants, the median score in the top tertile of interpersonal trust was 4.00 (Interquartile range within the tertile (IQR): 3.67–4.33), compared to 2.33 (IQR: 2.00–2.67) among those who were in the bottom tertile of interpersonal trust.

The multivariable adjusted models using backward stepwise selection identified additional variables to retain in the model as factors significantly associated with high interpersonal trust (Table 2). Of all the variables in the final model, optimism was the most strongly associated with interpersonal trust (OR_T3vsT1 = 5.75, 95%CI = 4.33–7.64). Additionally, persons who had voted in the last national election were more likely to experience high levels of interpersonal trust regardless of which party they chose (OR_{opposition party voters} = 1.39, 95%CI = 1.02–1.89 and OR_{governing party voters} = 1.61, 95%CI = 1.23–2.11), compared to non-voters. Perspective taking (OR_T3vsT1 = 1.46, 95%CI = 1.11–1.91) and being more extroverted (OR_T3vsT1 = 1.99, 95%CI = 1.53–2.59) and more agreeable (OR_T3vsT1 = 1.95, 95%CI = 1.46–2.61) was also associated with higher level of interpersonal trust. Similarly, individuals who scored higher on complexity thinking exhibited higher level of interpersonal trust (OR_T3vsT1 = 1.32, 95%CI = 1.01–1.72). In contrast, variables that were significantly associated with low levels of interpersonal trust included not regularly participating in religious meetings (OR = 0·61, 95%CI = 0·44–0·84, versus participation at least monthly) and being more conscientious (OR_T3vsT1 = 0.68, 95%CI = 0.51–0.91). Further, being a current smoker (OR = 0.68; 95%CI = 0.53–0.87, versus never smoking) or having sleep problems more than 5 times a week (OR = 0.48; 95%CI = 0.36–0.66, versus none) were both associated with significantly lower level of interpersonal trust. Lastly, scoring high on COVID-19 conspiracies was a significant predictor of low interpersonal trust (OR_T3vsT1 = 0.53; 95%CI = 0.41–0.69).

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Table 2. Factors cross-sectionally associated with interpersonal trust in the D-A-CH region (N = 3,065).

https://doi.org/10.1371/journal.pone.0286488.t004

Although results differed slightly by gender (see S1 and S2 Tables) and country (see S3–S5 Tables), patterns of associations remained largely unchanged in these substrata.

Discussion

In the present sample of adult residents of the D-A-CH region, we found that optimism, complexity thinking, as well as several sociodemographic and lifestyle factors strongly correlated with level of interpersonal trust. Those associations demonstrated independence in multivariable models (e.g., with mutual adjustment of the variables for one another). Further, although we observed some variation in levels of interpersonal trust reported by gender and country of residence, the association of other variables with level of interpersonal trust remained largely consistent across substrata defined by those factors. With the exception of complexity thinking, for which, to our knowledge, no prior evidence existed, earlier literature on predictors of interpersonal trust among different populations is mostly in line with our findings.

Age-related changes in trust behavior have previously been documented by cross-sectional studies [21], and findings largely agree with our results suggesting that interpersonal trust increases with older age. This is further substantiated by neuroscience, which has linked certain age-related changes in brain circuitry to changes in trust behavior [22]. Similarly, gender differences in interpersonal trust behavior have long been noted, suggesting that men have higher trust than women due to their lower sensitivity to risk and betrayal. A recent study using functional magnetic resonance imaging (fMRI) to distinguish neural signatures of trust by gender corroborates these findings [23]. In our data, we also observed slightly higher levels of interpersonal trust among the participants from Switzerland, versus Germany and Austria. Other studies have described variation in interpersonal trust across regions [24], but reasons are complex and they need to be put into the context of the respective political and societal context of a given region that could modify known associates of interpersonal trust (e.g., income). For example, one might speculate that the higher levels of trust observed in Switzerland could be related to the higher average income in Switzerland; nonetheless, our results were robust to adjustment for a variety of socio-demographic variables across the three D-A-CH regions. Higher education [25] and income [26] are two other well established predictors of higher interpersonal trust; reassuringly, our findings are very consistent with those earlier reports. Also in line with our findings, trust and work satisfaction have previously been shown to positively correlate; however, the direction of the relationship is less clear given that higher trust may induce higher engagement at work [27] and not the other way around. Longitudinal data would be needed to clarify the direction of this association; we are unable to contribute this insight with our cross-sectional survey.

Apart from the noted demographic factors, other traits and attributes emerged in our study as associated with interpersonal trust. Optimism exhibited the strongest association with trust, suggesting that persons with higher levels of optimism are more than 5 times as likely to also exhibit higher interpersonal trust, compared to those with comparatively low levels of optimism. The temporal directionality of this association cannot be addressed due to the cross-sectional nature of our data; but previous studies have suggested a strong correlation between these two factors [11, 28, 29]. Jones [30] argued that trust constitutes an attitude of optimism, suggesting that optimism antecedes trust. Additional factors that were positively associated with trust in our data can be viewed as facets of trust (e.g., the trait of agreeableness) and cheerfulness (e.g., the trait of extroversion).

Few previous studies examined the association between voting behavior and trust, and the scarce literature that exists suggests that generalized interpersonal trust is associated with a higher willingness to engage in political and social activities [5, 31]. Alternatively, a study by Albright et al. [32] implicated smoking behavior to be associated with voting behavior, and thus, indirectly with trust. Interestingly, we too, found a significant association between both current smoking and non-voting with lower levels of trust. A possible causal explanation relates to the increasing stigmatization of smokers [33], which, through the close alignment of social and political engagement, could lead to lower voting behavior among smokers. That we also see lower levels of trust among those not participating in religious meetings could be another indication of such social disengagement. However, of note, in our analyses, each of these factors (smoking, non-voting, no participation in religious meetings) remained significant after mutual adjustment. Further, we found that scoring higher for belief in COVID-19 conspiracies was associated with lower interpersonal trust,—a finding that has consistently been shown during the COVID-19 pandemic [34, 35].

An interesting and potentially important finding of our study is that increasing ability for complex thinking was associated with higher levels of trust. Complexity thinking allows one to see deep similarities between the structures and dynamics of different phenomena. While trust itself represents a highly complex construct, the ability to engage in complex thinking was positively associated with trust in our analyses (and in contrast, being more conscientious, a facet of achievement striving [36], was inversely associated with trust). There is little to no literature on the relationship between complexity thinking, optimism, and interpersonal trust; however, in an increasingly complex and crisis-ridden world, it seems of utmost importance to pay careful attention to these aspects beginning even in the earliest years in school. Our data may help to encourage the development of effective strategies for nurturing these edifying traits from an early age.

Lastly, sleep problems were significantly linked with lower interpersonal trust in our study. Krueger and Meyer-Lindenberg [9] postulated trust as a central component to mental health disorders such as schizophrenia or borderline disorder; and sleeping difficulties frequently co-occur with health disorders, particularly mental health problems [37]. In contrast, optimists have been shown to sleep better [12]. In a South Korean/Taiwanese study of social determinants of self-reported sleep problems, the authors found interpersonal trust not to be significantly associated with sleep problems in their multivariable models [38]. This contrasts somewhat with our findings, though cultural differences cannot be excluded. It would seem interesting to examine effect modification of the association between sleeping problems and trust by mental health disorders, such as depression, but we did not collect information on mental health in our survey.

Our study has several strengths, including the large number of socio-demographic variables and personality traits, as well as newly developed complexity and COVID-19 conspiracy belief scores, that were assessed; the population representativeness of our large sample, which allowed for good generalizability of our results and comparisons among three different German-speaking countries; and the use of multiple validated tools to assess a variety of personality traits and psychological constructs. Some limitations also deserve noting. The cross-sectional nature of our survey precludes assessing the temporality of several associations (e.g., whether work satisfaction increases trust or vice-versa). Even though we used validated instruments where possible, the purely self-reported nature of some items (e.g., sleeping difficulties) may have introduced some misclassification bias. However, our main outcome measure to assess interpersonal trust was the validated “Kurzskala für interpersonales Vertrauen” (KUSIV3). This instrument has subsequently been shown to correlate well with other trust scales such as e.g., the Interpersonal Trust Scenario Questionnaire (ITSQ) (29). As noted earlier, we did not query information on mental health status, which prohibited us examining different strata (with or without mental health conditions). That our survey was implemented in German precluded participation of non-German speaking participants, especially in Switzerland but also in Germany and Austria and limited the generalizability of our findings to these population groups.

In summary, our study suggests several factors that could serve as potential targets for interventions aimed at strengthening interpersonal trust and thereby people’s engagement in society. We found several previously established associates of interpersonal trust, such as age, gender, education and work-related variables; but also several new and potentially actionable associations, most importantly optimism (which can be increased through simple interventions [39]) and complexity thinking. The latter may provide an intervention point to implement complexity thinking training in schools and universities, as the world is facing numerous crises with increasing needs for complex thinking and higher levels of engagement in society, which in turn relies on higher interpersonal trust.

Supporting information

Acknowledgments

We are grateful to the survey respondents for their participation.