A new study published in Current Psychology shows that young adults who support inequality between social groups and hierarchies tend to have less gray matter in the anterior insula, a brain region involved in empathy and emotional processing. The research, led by Jesús Adrián-Ventura, a professor at the University of Zaragoza, suggests that social and political worldviews may be partly linked to basic differences in motivation and emotion.
Social dominance orientation is a personality trait that reflects how much someone prefers inequality between groups over equality. People who score highly on this trait see society as naturally divided into superior and inferior groups and often support systems and policies that maintain these hierarchies, while those with lower scores prefer equal relationships between groups.
To investigate how this mindset relates to the brain, the researchers focused on the anterior insula, a part of the brain located deep in the lateral sulcus, beneath the frontal, parietal and temporal lobes. The anterior insula plays a central role in detecting bodily sensations and processing other people's emotional experiences. Previous psychological research had shown that people who strongly favor social dominance tend to report lower empathy overall, while another study linked social dominance orientation to a positive orientation toward rewards, meaning the psychological drive to actively pursue rewards.
A previous brain imaging study from 2023 involving adults in Singapore had found that higher social dominance scores were associated with less gray matter in the anterior insula and ventromedial prefrontal cortex, as well as more gray matter in the amygdala. However, some of these results were not confirmed using more rigorous analytical methods.
Adrián-Ventura and his colleagues wanted to test these findings again using improved techniques in a Spanish sample. The new study involved 99 healthy young adults aged 18 to 30 (62 women and 37 men) from a university campus in Spain. After excluding two participants because of extreme brain volume values, the neuroimaging analyses were conducted on 97 people. Participants completed an eight-item questionnaire on social dominance and another 24-item questionnaire on reward sensitivity, while brain scans were performed using high-resolution 3-tesla MRI. The researchers analyzed predefined regions of interest based on a standardized anatomical atlas, adjusting their calculations for age and head size.
The results showed that higher social dominance scores were associated with a smaller volume of gray matter in both the left and right anterior insula. The correlations were r = -0.30 for the left side and r = -0.29 for the right side, after adjusting for age and head size, and remained statistically significant even with a stricter threshold to account for multiple comparisons.
However, an exploratory whole-brain analysis found no significant associations anywhere, including in the insula; the link appeared only in the targeted analysis of the three predefined regions. Unlike the Singapore study, the researchers found no evidence linking social dominance to the amygdala or ventromedial prefrontal cortex. The correlations with the amygdala were small and not statistically significant, as was the correlation with the ventromedial prefrontal cortex.
Adrián-Ventura said that the absence of these associations was somewhat unexpected, as it contradicts the only previous neuroanatomical study. He added that the difference might be due to cultural differences, since the earlier study involved Singaporean Chinese adults, but the authors note that this explanation is speculative and that methodological differences could also play a role.
At the behavioral level, social dominance orientation was positively correlated with reward sensitivity, with a moderate correlation of r = 0.33. In statistical models predicting social dominance scores from anterior insula volume, age and head size, adding reward sensitivity increased the proportion of explained variance by about 10 to 11 percentage points; the full model explained up to 29% of individual differences among the 95 participants with complete data.
Adrián-Ventura emphasized that the results are quite substantial and that structural brain characteristics and personality traits are largely heritable and stable throughout life, suggesting that preferences for social hierarchies may not simply be temporary attitudes but may have roots in early developmental and genetic factors.
David V. Smith, an associate professor of psychology and neuroscience at Temple University who was not involved in the study, noted that the findings offer interesting starting points for investigating how motivational processes relate to these attitudes, but that he would like to see larger studies before considering the pattern well established. He also added that a smaller volume of gray matter does not automatically mean less empathy, and that greater general sensitivity to reward does not prove that someone finds inequality rewarding. He described the anatomical findings as preliminary, pointing out that 97 participants is a relatively small sample for linking individual differences in brain anatomy to psychological traits, and that he would like to see whether the association holds when sex is included in the models.
The study builds on previous work by the same team, which had shown that higher scores on a facet of left-wing authoritarianism involving hostility toward authorities were associated with a thinner cortex in the right anterior insula. The authors suggest that the anterior insula findings may reflect differences in social and emotional processing: because this region helps people attune to others' emotional distress, a smaller volume of gray matter could be associated with reduced empathy, which, combined with a stronger drive toward rewards, could make it easier to support social arrangements in which some groups dominate others.
However, the study is cross-sectional and cannot prove a causal relationship, the sample consists mainly of young Spanish university students, and the research was not preregistered. The researchers plan future longitudinal studies using samples that allow the findings to be generalized more widely. Smith suggests measuring empathy and preference for relative advantage to determine whether people value their group's superiority even when the absolute benefit remains the same.
Comments