Individual differences in facial expressivity: Social function, facial anatomy and evolutionary origin

    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.

    FACEDIFF project overview showing the role of each work package (WP). WP1 will test how individual differences in facial expressivity are related to individual social networks. WP2 will assess population variance in facial musculature in humans and macaques. WP3 will test how individual differences in facial expressivity (underpinned by musculature) are related to social networks in a nonhuman primate comparison. WP4 will integrate the data from the three WPs to determine the overarching relationship between anatomy, expressivity and social network formation in an evolutionary context.

    FACEDIFF is led by Professor Bridget Waller based at Nottingham Trent University, in collaboration with the University of Liverpool, University of Portsmouth, and the Medical Research Council Centre for Macaques. 

      The FACEDIFF Team

      Dr Eithne Kavanagh

      Research Fellow

      - Leading human projects

      Dr Jasmine Rollings

      External collaborator

      Alisa Balabanova

      PhD student

      Andrew Buckee

      Research Assistant (WP1 + WP3)

      Human behaviour team


      Dr Claire Tierney 

      Lecturer in Anatomy

      - Collaborator for human

      anatomy projects

      Prof Anne Burrows 


      - External collaborator for

      primate dissections

      Anatomy team


      Dr Jamie Whitehouse

      Research Fellow

      - Leading non-human primate projects

      Pantelina Syrimi

      Research assistant

      Dr Jerome Micheletta

      Reader in Animal Behaviour

      - External collaborator for primate research

      Dr Claire Witham

      Head of Scientific Programs

      MRC Centre For Macaques

      - External collaborator for primate research

      Dr Clare Kimock

      Research Fellow (WP2+3)

      - Leading macaque field research and supporting macaque anatomical work

      Prof Bridget M. Waller

      Professor of Evolution & Social Behaviour

      - Principal Investigator

      - Grant Awardee

      Olivia O'Callaghan

      PhD student

      Kerensa Rees

      Research Assistant

      Dr Peter Clark

      Lecturer in Psychology

      - External collaborator

      Rachel Robinson

      PhD student

      Primate behaviour team


      Ongoing Projects

      Can facial expressivity be measured by self-report?

      Individuals seem to differ in overall facial expressivity, which may reflect how successfully individuals use facial expressions as social tools. Measurement of spontaneous facial expression, however, can be time consuming. In our study we develop a reliable and valid instrument to measure individuals’ facial expres-sivity by administering a 38-item preliminary question-naire to 400 participants online. We will test the efficacy of this questionnaire in reflecting participants’ actual use of facial expression, by comparing their scores to their facial movement in a video call designed to mimic a naturalistic social interaction, along with other tests of reliability and validity. Our final instrument will facilitate research testing the social function of facial expression and the impact of individual differences in overall expressivity.

      Automated measures of expressivity in macaques

      Manual observation of subjects and any subsequent video coding can be highly demanding on researcher time and resource. As a consequence, the amount of data able to be collected and processed is limited. Here, using a facial landmark tracking system developed in DeepLabCut, we attempt to automatically extract facial movements in captive macaques. If successful, this approach will allow us (and others) to potentially produce bigger and richer expressivity datasets. This sub-project is lead by Dr. Claire Witham and her students. 

      Signal value of stress behaviour

      Physiological and psychological stress is often accompanied with nonverbal behaviour (self-directed displacement behaviour). The function of this behaviour is not well understood but is often assumed to be read by others as a cue to stress. Here, we test whether such behaviours are reliable indicators of stress in humans. Both behaviour and self-reported stress ratings given by stressed participants were positively associated with the stress ratings provided by observers and therefore, such behaviours can provide reliable information to others. Observers differed in their ability to detect stress from nonverbal cues which may relate to real-world social skills as it appears associated with the individuals’ social network size.

      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.

      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.


      Peer reviewed publications

      Kavanagh, E., Whitehouse, J., & Waller, B. (2022). The face in everyday social interaction: social outcomes and personality correlates of facial behaviour. Psyarxiv. 10.31234/osf.io/7tbyr

      Clark, P. R., Waller, B. M., Agil, M., & Micheletta, J. (2022). Crested macaque facial movements are more intense and stereotyped in potentially risky social interactions. Philosophical Transactions of the Royal Society B, 377(1860), 20210307.

      Rollings, J., Micheletta, J., Van Laar, D., & Waller, B. M. (2022). Personality traits predict social network size in older adults. Personality and Social Psychology Bulletin, 01461672221078664.

      Kavanagh, E., Kimock, C., Whitehouse, J., Micheletta, J., & Waller, B. M. (2022). Revisiting Darwin's comparisons between human and non-human primate facial signals. Evolutionary human sciences, 4, e27.

      Whitehouse, J., Milward, S. J., Parker, M. O., Kavanagh, E., & Waller, B. M. (2022). Signal value of stress behaviour. Evolution and Human Behavior.


      Waller, B. M., Kavanagh, E., Micheletta, J., Clark, P. R., & Whitehouse, J. (2022). The face is central to primate multicomponent signals. International Journal of Primatology, 1-17.


      Rollings, J., Micheletta, J., Van Laar, D., & Waller, B. M. (2022). Personality traits predict social network size in older adults. Personality and Social Psychology Bulletin, 01461672221078664.

      Julle-Danière, E., Whitehouse, J., Vrij, A., Gustafsson, E., & Waller, B. M. (2020). The social function of the feeling and expression of guilt. Royal Society open science, 7(12), 200617.

      Jobs and positions opportunities

      Marie Curie (MSCA) Postdoc Fellowships are now open for applications


      We always welcome enquires from potential PhD students

      Anyone interesting in joining the team, contact us! 

      Research Assistants

      We are happy to consider individuals for short term research assistant work if your skills are aligned with the project, and there is work currently available. This could include data collection, coding, or conducting complementary research projects. Please send serious enquiries along with your CV to bridget.waller@ntu.ac.uk or contact@facediff.co.uk!

      Contact Us

      For more infomation, please email bridget.waller@ntu.ac.uk, or through the form below



      This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864694)

      The study Signal Value of Stress was funded in part by a British Academy small research grant SRG18R1\180883