Dissecting the midlife crisis: disentangling social, personality and demographic determinants in social brain anatomy

Humans are inherently social beings, maintaining a strong desire for social connection throughout life. Midlife (ages 40-70), a period marked by significant social transitions (e.g., new leadership roles, caregiving responsibilities), is associated with structural remodeling in the medial prefrontal cortex (mPFC), a brain region crucial for social cognition. Existing literature links mPFC structure to social behavior, personality, and demographics, but these factors are often studied in isolation. This study aims to comprehensively investigate the unique contribution of each factor to mPFC structure and social brain morphology in a large, diverse sample.

Previous neuroimaging studies have shown age-related changes in brain structure, particularly in the frontal and prefrontal cortices. However, the interpretation of changes in gray matter volume remains complex, with increases potentially indicating enhanced neural density or decreased volume reflecting computational efficiency gains. The mPFC has been implicated in various aspects of social cognition, including self-concept and mentalizing (understanding others' mental states). Furthermore, personality traits (e.g., neuroticism, conscientiousness, extraversion) and demographic factors (e.g., socioeconomic status, income, occupation) have been linked to mPFC structure and social behavior. These previous studies, often limited by small sample sizes and focusing on specific factors, highlight the need for a large-scale, integrated approach.

This study leveraged the rich data from the UK Biobank, analyzing T1-weighted structural brain MRI data and 40 lifestyle traits from approximately 10,000 middle-aged participants (ages 40-69). The traits were categorized into three domains: social exchange (e.g., household size, friendship satisfaction, romantic partners), personality profile (e.g., morningness-eveningness, neuroticism, happiness), and demographic status (e.g., income, occupation, education). A fully Bayesian framework, employing probabilistic multiple regression, was used to disentangle the unique contribution of each trait to the variation in gray matter volume within 36 regions of a social brain atlas. A partial correlation analysis was also conducted to further isolate unique trait effects. The analysis accounted for sex differences and examined the joint effects of age and specific traits.

The study revealed that several social traits, particularly those related to close relationships and living arrangements, strongly influenced social brain volume. Sharing a home with others was a consistent top predictor across numerous brain regions. In women, household size, friendship satisfaction, and the number of romantic partners showed significant effects. In men, being a morning person was strongly associated with several brain regions. Demographic indicators, particularly high income (in men) and health satisfaction (in women), also significantly predicted mPFC gray matter volume. The partial correlation analysis provided a more nuanced picture, revealing a wider range of trait associations and further highlighting the sex differences in the relationship between brain structure and lifestyle. The inclusion of age in the analysis demonstrated sex-specific patterns of joint effects, with different lifestyle indicators most strongly linked to brain region variation in men compared to women.

This study provides novel insights into the complex interplay between social brain anatomy and various aspects of midlife experiences. The findings support the idea that the quality and composition of social networks, and social status, strongly influence social brain morphology during midlife, suggesting that maintaining close relationships and achieving a certain degree of social standing may have long-term benefits for brain health. The sex-specific differences observed emphasize the importance of considering gender in studying the social brain. The significant contribution of daily routines and well-being highlights the role of lifestyle choices and personality in shaping brain structure.

This large-scale study demonstrated that social and demographic factors, more than personality traits, significantly predicted variation in social brain volume during midlife. Household size, social support, and income were consistently found to be strong predictors of variation across many regions. Future research could investigate the causal directionality of these relationships, potentially using interventions such as TMS. Furthermore, longitudinal studies are needed to understand how these associations change over time and their contribution to aging.

The study is correlational in nature, limiting inferences about causality. The UK Biobank traits are not always perfectly aligned with traditional psychological measures, and the scope of indicators may not capture the full complexity of environmental and life experiences. Additionally, the automated quality control procedures used, while widely accepted, may have introduced some minor inaccuracies.