We use facial expressions to communicate. This kind of relatively simple language plays an important role in our social life. From person to person, however, it can differ greatly. It can also be affected by specific medical or psychological disorders. However, the social function, anatomy and evolution of these individual differences in facial expressions have yet to be understood. The 5-year (2020-2025) ERC-funded FACEDIFF project will pioneer an interdisciplinary study investigating the cause of differentiation in facial expression and how this results in benefits or costs in an individual’s social engagement. FACEDIFF will use various (psychological, anatomical and cross-species) methods to measure differences in individual production and perception of facial expressions, tracing their evolutionary determinants.

First, individual variation in production and perception of facial expressions will be measured via laboratory experiments and in relation to social network size and quality. Second, variation in human facial musculature will be documented through cadaveric dissection and existing MRI databases. Third, facial expressivity will be examined in a primate model to determine whether patterns are unique to humans. This project will be the first to provide a comprehensive and interdisciplinary perspective on individual differences in facial expression and will stimulate new theories on the function and evolution of individual differences in humans.

Primate behaviour team

WP3

Facial expressivity benefits top ranking male macaques

Social living affords primates (including humans) many benefits. Communication has been proposed to be the key mechanism used to bond social connections, which could explain why primates have evolved such expressive faces. In this study, we compare social network data and expressivity indices between captive macaque groups in uniform physical and social environments. We find that facially expressive dominant male macaques had more cohesive social groups and were more socially connected within their groups. These findings show that inter-individual differences in facial expressivity are related to differential social outcomes at both an individual and group level. More expressive individuals occupy more advantageous social niches, which could help explain selection for complex facial communication in primates.

Whitehouse, J., Clark, P. R., Robinson, R. L., Rees, K., O’Callaghan, O., Kimock, C. M., ... & Waller, B. M. (2024). Facial expressivity in dominant macaques is linked to group cohesion.Proceedings of the Royal Society B,291(2027), 20240984.

Linking individual variation in facial musculature to facial behaviour in rhesus macaques

Primates use movements of the face, produced by muscles of facial expression, to communicate. Facial muscles exhibit marked inter-individual variation, but how individual differences in these muscles impact facial behaviour is unknown. In this study, we are investigating intra-individual relationships between facial muscle morphology and facial behaviour to capture within-species patterns of covariation between facial muscle form and function, combining anatomical dissections with ethological observations. Our results will highlight how facial musculature may underpin observed facial behaviour in a non-human primate species and help us understand the form and function of within-species variation in facial musculature.

Maternal care predicts facial expression processing in macaques (led by Olivia O'Callaghan)

Facial expressions are common across mammals and are essential for social communication. In humans, a rich early social environment is important for the appropriate development of facial expression processing. Whether other animals are similarly reliant on social input for facial expression development, or have a more fixed inherent system, is unknown. Here, we investigated how maternal care influences facial expression processing skills in rhesus macaques (Macaca mulatta). We conducted three experiments quantifying facial expression processing and examined performance in relation to historical maternal data and across age. Facial expression processing skill was predicted by positive social contact with the mother during infancy and increased with age until adulthood. Our findings provide the first evidence that early social input, specifically maternal care, enhances facial expression processing skills in non-human animals. This challenges the long-standing notion that facial expression processing is hard-wired and innate, and instead suggests a flexible system responsive to local conditions.

Facial expressivity predicts social popularity (led by Alisa Balabanova)

Complex mechanisms have evolved in humans to aid uniquely close and cooperative group living. Facial expressions are produced frequently during interpersonal communication and can express internal states, punctuate language and demonstrate attention to others. However, whether use of facial expressions has adaptive value by leveraging a social advantage and aiding group cohesion is not well evidenced. Here, we tested the relationship between facial expressivity and the formation of interpersonal connections in an informal, online group setting. Participants (previously unacquainted) met in groups of three or four (N=256; in 72 groups) in an unstructured online video call interaction.  We extracted facial expressivity during 5min-long interactions based on automated Facial Action Coding Scheme analysis and obtained partner perception measures by self-report.  Using social network analysis, we calculated network centrality based on participants’ liking judgements as a measure of social popularity.  Participants were also rated on attractiveness by 620 independent raters. Results indicated that more facially expressive individuals occupied more central positions within their group, and were perceived as warmer and more cooperative. Interestingly, attractiveness did not predict popularity. Facial expressivity may enhance popularity by demonstrating increased behavioural predictability and/or honest signalling, characteristics important for group living. Our findings suggest, therefore, that facial behaviour aids the formation of social ties and has evolved as a key adaptive mechanism in group social cohesion.