Introduction

Humans commit to political, societal, and environmental causes because they are affected by injustice inflicted on others. However, there is enormous heterogeneity in how sensitive people are to vicariously experienced injustice. Some individuals are outraged when witnessing injustice to others, while others watch injustice callously [1]. Previous studies have documented the individual heterogeneity in vicarious injustice sensitivity [2,3], and highlighted its strong predictive power for actual social outcomes. For example, vicarious injustice sensitivity is related to the propensity of altruistic sharing [4–6], and altruistic punishment [7], and has an impact on moral judgments [8]. This evidence demonstrates that understanding heterogeneity in vicarious injustice sensitivity is essential for understanding individual differences in social outcomes. However, so far, the sources of individual heterogeneity in vicarious injustice sensitivity remain unclear.

Recent applications of brain morphometry indicate that individual differences in brain structure can be useful in understanding individual differences in personality traits [9–11], social behaviors [12–15], and skills [16–19]. We therefore conjectured that variables reflecting relatively stable neuroanatomical individual differences, such as gray matter (GM) volume and cortical thickness, may help to predict individual differences in vicarioius injustice sensitivity.

Previous brain imaging studies have studied the functional neural responses during the vicarious experience of injustice to others, for example when others receive an unjustly small share of an available monetary amount [20,21]. According to their results, observing injustice to others is associated with neural activation in the mid and anterior insular cortex [20,21], in line with a large body of work that has linked this region to other vicarious experiences such as pain [22,23], embarrassment [24,25], or ostracism [26,27]. Based on these findings, we hypothesized that the structural differences of the mid and anterior insular cortex may provide a plausible neuroanatomical basis for situation- and context-independent individual differences in vicarious injustice sensitivity.

To test this assumption, participants imagined and rated scenarios in which others were subjected to injustice, taken from the well-established Justice Sensitivity Scale [28]. In an independent MRI session, we acquired structural images (T1-weighted MRI scans), which were analyzed with voxel-based morphometry (VBM), a whole-brain technique capable of discovering subtle, regionally specific changes in gray matter volume and Freesurfer image analysis suite, a technique that allows to disentangle the effects of cortical thickness and cortical surface.

Results

To determine regions whose gray matter volume is associated with the individual injustice sensitivity, the individual vicarious injustice ratings were regressed against the gray matter volume images using VBM. Age and brain size were included as control variables [29,30]. Family-wise error (FWE) whole-brain cluster correction was used to control for multiple comparisons. In line with our assumptions, the results showed a significant positive correlation between the gray matter volume of the bilateral mid insular cortex and the vicarious injustice sensitivity ratings, left insula, center-coordinate, x = -33, y = -6, z = -5; p < 0.05, corrected, r = 0.528, R² = 0.28; right insula, center-coordinate, x = 34, y = 10, z = 17, p < 0.05, corrected, r = 0.548, R² = 0.30 (Fig 1A and 1B). The larger the gray matter volume of the bilateral insula the greater the participant’s sensitivity to injustice inflicted on others (Fig 1C and 1D).

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Fig 1. Observer Justice Sensitivity is positively associated with insular gray matter volume.

The gray matter volume (GMV) is adjusted for age and brain-size. (A) Effect shown in sagittal view of the left insula (at p < 0.05, FWE-corrected for the whole brain, for display purpose depicted at p < 0.001 uncorrected). (C) Scatter plot of Observer Justice Sensitivity (JS) against the z-standardized GMV of the significant cluster in the left insula. (B) Same as (A) for right insula. (D) Same as (C) for right insula.

https://doi.org/10.1371/journal.pone.0167538.g001

The correlation analysis did not reveal any other significant neural effects, i.e., the mid insular cortex was the only brain structure that was significantly linked to vicarious injustice sensitivity.

To test for the impact of gender, we conducted the same analysis with gender as additional co-variate. The results confirmed the significant positive correlations between the gray matter volume of the bilateral mid insular cortex and the vicarious injustice sensitivity ratings (left insula, p < 0.05, corrected, r = 0.516, R² = 0.27; right insula, p < 0.05, corrected, r = 0.538, R² = 0.29). This indicates the validity of our findings irrespective of subjects’ gender.

In order to obtain a more fine-grained understanding of the underlying structural differences driving the relationship between insular gray matter volume and vicarious injustice sensitivity, we additionally performed a vertex-based structural analysis with the Freesurfer image analysis suite (see material and methods for details). Freesurfer allows for looking at the two components of gray matter volume separately, i.e. cortical thickness and cortical surface. This analysis is thus able to provide evidence of whether the differences observed in gray matter volume of the insular cortex are due to differences in cortical thickness, cortical surface, or a combination of both. In order to answer this question, we conducted linear regression analyses with either the cortical thickness or the cortical surface component of the insular cortex as the dependent variable and the vicarious injustice sensitivity ratings as independent variable, controlling for age and intracranial brain volume. Results showed that only cortical thickness was positively associated with the Observer Justice Sensitivity (right Insula: r = 0.392, p = 0.003; left Insula: r = 0.366, p = 0.007), whereas cortical surface was not (right Insula: r = 0.137, p = 0.323; left Insula: r = -0.098, p = 0.482).

An additional analysis that included gender as co-variate revealed comparable results for cortical thickness (right Insula: r = 0.394, p = 0.003; left Insula: r = 0.366, p = 0.007) and surface area (right Insula: r = 0.134, p = 0.340; left Insula, r = -0.103, p = 0.464), indicating that the results are not altered by subjects’ gender.

Discussion

These findings demonstrate a link between neuroanatomical brain structure and vicarious injustice sensitivity. The larger the GM volume and thickness of a person’s bilateral mid insular cortex, the higher this person’s propensity to respond sensitively to injustice inflicted on others. Previous studies have shown that vicarious injustice sensitivity varies enormously among individuals, and is relatively stable across time within a person [2,3], without accounting for this heterogeneity and temporal stability. The present study reveals inter-individual variation in the gray matter volume and thickness of the insular cortex, which, as we show here, can account for the individual heterogeneity in vicarious injustice sensitivity.

In contrast to previous studies that assessed neural responses “online” when subjects actually witnessed situational and context-dependent injustice, our study uses an “offline” measure of vicarious injustice sensitivity that is unadulterated by context or task demands [16]. Our finding that the neuroanatomy of the mid insular cortex reflects this “offline” measure thus captures a person’s predisposition to vicariously respond to injustice inflicted on others.

Based on anatomical connectivity studies it is well established that the insula is the first cortical target of ascending interoceptive [31] and viscerosensory [32] inputs. Given this evidence it has long been considered to be important for the generation of subjective feelings that guide decision making [33]. Remarkably, such subjective feelings are not only generated by first-hand inputs, but also by sensory and emotional experiences observed in others [20–22,24–27]. Aggregating these results, recent models propose that the mid and anterior insular cortex plays a central role in assigning relative salience to direct and vicarious inputs, and, as a result, determines the emotional and behavioral relevance of first-hand and vicarious experiences [34].

In the context of our findings, this might indicate that individuals with a larger mid insular cortex experience unjust treatment to others more saliently, which leads to a higher degree of vicarious injustice sensitivity. This interpretation is supported by the results of functional MRI studies that show an increase of functional neural response in this region in response to witnessed injustice [20,21]. The finding that previous functional “online” measures and our structural “offline” measures converge in the same region might suggest that the mid insular cortex integrates the state (captured by functional activation) and trait (captured by brain structure) dimensions of vicarious injustice sensitivity.

In this case, a person’s individual vicarious injustice sensitivity trait, reflected by the mid insula volume and thickness, should predict the magnitude of the “online” functional response during actual vicarious injustice experiences. Vice versa, inspired by studies that show the impact of long-term social factors on neuroplasticity [35,36], it is also conceivable that an enduring state of vicarious injustice experiences alters a person’s injustice sensitivity trait, captured by the mid insula GM volume and thickness.

Our results show that cortical thickness, but not cortical surface area is associated with a person’s injustice sensitivity trait. Cortical thickness and surface area are considered independent neuroanatomical traits that are likely to be influenced by different factors during brain development [37] and have different origins [38]. While cortical surface area increases during late fetal development due to cortical folding, cortical thickness alters dynamically across the entire life span as a consequence of experience, training and disease. This implies that the individual differences in cortical thickness of the insular cortex we observed might be less rooted in early cortical development, but rather be driven by different life experiences (e.g. experiences of injustice) and moreover, could be altered by training (e.g. empathy training).

It is noteworthy that previous research has associated the mid and anterior insular cortex to the processing of empathy [22], i.e., the affective sharing and understanding of both pleasant and unpleasant emotions [39]. Our finding that the neuroanatomy of the same structure predicts individual differences in vicarious injustice sensitivity is in line with recent work that has linked trait empathy to a person’s willingness to punish witnessed injustice [40], and supports models that assume a close relationship between empathy, justice and morality [41,42].

In sum, our study provides the neuroanatomical basis for vicarious injustice sensitivity, and thereby an account for its enormous individual heterogeneity. Based on recent models [34] we propose that the gray matter volume and thickness of the mid insular cortex might reflect the relative salience assigned to injustice inflicted on others, and thus determines a person’s predisposition for vicarious injustice sensitivity.

Materials and Methods

Supporting Information

Acknowledgments

This project was supported by a grant to D.K. by the Swiss National Science Foundation (PP00P1_146317).