Horváth K., Martos J, Mihalik B., Bódizs R.: Is the social brain theory applicable to human individual differences? Relationship between Sociability personality dimension and brain size, International Journal of Psychophysiology Volume 77, Issue 3, 2010

Klára Horváth¹, János Martos², Béla Mihalik², Róbert Bódizs^1,3

1 Institute of Behavioural Sciences, Semmelweis University, Budapest, Hungary

2 Department of Neuroradiology, National Institute of Neuroscience, Budapest, Hungary

3 HAS–BME Cognitive Science Research Group, Hungarian Academy of Sciences, Budapest, Hungary

Keywords: Social brain theory, Sociability, brain volume, MRI

Introduction

A plausible explanation of the fact that primates have larger brains than it is expected from their body sizes compared to other vertebrates is provided by the social brain theory. According to this hypothesis, the unusual brain size is due to the adaptation to a more complex social life. Many authors provide a reflection on social brain theory at a behavioural level but these studies concentrate on interspecies differences. The few results regarding humans show that theory of mind correlates with the volumes of temporal and frontal lobes. The aim of this study is to examine whether the individual differences in human brain volumes are connected with Sociability personality dimensions that is associated with individuals’ social lives including their social skills. We hypothesize that the level of Sociability is positively related to the volumes of several cortical structures including the frontal and the temporal lobes. As far as we know the neuroanatomic correlates of Sociability have not been studied yet. There are several studies which examined the NEO-PI-R Extraversion, but the results are inconsistent. We assume that this inconsistence is caused by the combination of other factors (Activity, Sensation Seeking, Impulsivity).

Methods

Twenty-three healthy adults (16 men, 7 women) participated in this study. Sociability was assessed by the Hungarian version of the Zuckerman-Kuhlman Personality Questionnaire form III. The volumes of brain structures were measured by analysing MRI scans with the HAMMER (Hierarchical Attribute Matching Mechanism for Elastic Registration) software package. This method can automatically separate grey matter, white matter, cerebrospinal fluid, as well as different brain structures by adopting an anatomical template to individual anatomies. Partial correlation coefficients were computed between the volumes of the brain structures and Sociability scores with the control of height.

Results

All of our primary hypotheses have been supported by the results, as Sociability score correlated positively both with the whole and the regional brain volumes controlling for height. Specifically, positive correlations were observed between the right frontal lobe (r[23]=0,508; p=0,016), the left frontal lobe (r[23]=0,478; p=0,024), the right temporal lobe (r[23]=0,642; p=0,001) and the left temporal lobe (r[23]=0,644; p=0,001). Furthermore, we found a positive correlation between Sociability score and the volume of the cerebrum (r[23]=0,532; p=0,011).

Discussion

Our findings support the presumption that increased and more complex social life could be associated with increased brain volumes, and suggest that the social brain theory can be extended from an interspecies level to an interindividual one. Further studies are needed to decide whether Sociability as a personality trait or some specific sociability-related skill is responsible for the increased brain size in humans.