Universiteit Leiden

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Research project

Social decision making in humans and great apes

Efficiently responding to others’ emotions has great survival value, especially for social species, such as primates, who establish close, long-term bonds with group members. The closest living relatives to humans are the chimpanzee and the bonobo. Studying these species, and comparing them on the exact same emotion tasks, gives us indirect insight into the evolutionary trajectory of emotional behavior.

2014 - 2018
Mariska Kret

Leids Universitair Fonds (LUF)

Elise-Mathilde Stichting

KNAW Dobberke Stichting voor Vergelijkende Psychologie

Tobii Pro

Thermal Focus

Bonobos spend their lives in large social groups and for their survival have to rely on that group and its members. Observational research has shown that bonobos are very social and can adequately regulate their own and group members’ emotions, thereby often preventing conflicts from arising or solving them quickly. We recently demonstrated that bonobos, like humans, have heightened attention for emotional compared to neutral signals of conspecifics (Kret et al., 2016). With aid of the dot-probe task, a paradigm designed for testing attentional biases in humans but also useful for comparative studies (van Rooijen, Ploeger & Kret, in press), we demonstrated that bonobos were particularly drawn toward scenes showing other bonobos that were yawning, mating, or grooming, but not toward scenes depicting distressed bonobos, bonobos pant-hooting, playing, or handling food compared with neutral scenes (Kret, Jaasma, Bionda & Wijnen, 2016).

Like bonobos and humans, chimpanzees also live in social groups. The leader of the group is the dominant alpha male, which is unlike as in bonobos, where a female leads the group. In collaboration with Prof. Tetsuro Matsuzawa and Prof. Masaki Tomonaga, I study this species and compare their behavior to human. In our first project, we showed that chimpanees, like humans, synchronize their pupil size with conspecifics (see also: Role of Pupil-synchronization in trust). This shows that pupil mimicry is likely an evolutionary old phenomenon shared by our common ancestor. In a second project we investigated how chimpanzees compared to humans recognize each other. Previous research in chimpanzees already showed that they can recognize each other by the face but also by the behind. Other research showed that humans and at least some chimpanzees process faces holistically and recognize faces in the blink of an eye. This fast processing route gets disrupted when faces are presented upside down, a phenomenon dubbed ‘the face inversion effect’. In our study we for the very first time observed a ‘behind inversion effect’ in chimpanzees. Chimpanzees recognize each other from the behind immediately, and process this body part holistically, similarly to how humans process faces.

The orangutan is the only great ape species that lives semi-solitary and not in a social group. The orangutan has the longest childhood dependence on the mother of any animal in the world: the babies nurse until they are about six years old. The males generally remain solitary until they encounter a female who is receptive to mating. They will stay with this female for several days but will soon resume their solitary life. So it will be very interesting to investigate how orangutans, compared to other great apes and compared to us, respond to the emotions of conspecifics. Do they pay attention to conspecific’s emotions, do they empathize with them, do they mimic their expressions? And does it matter whether they are being confronted with a familiar individual, someone they know, versus someone they don’t? All these questions are on the research agenda and I’m thrilled to find their answers.


Van Rooijen, R., Ploeger, A., & Kret, M.E. (in press). The dot-probe task to measure emotional attention: A suitable measure in comparative studies? Psychonomic Bulletin & Review.  Download.

Kret, M.E. & Tomonaga, M. (2016). Getting to the bottom of processing behinds in humans and chimpanzees (Pan Troglodytes). The face inversion effect revisited. PlosOne, 11(11): e0165357. I = 3.730. Download.

Kret, M.E., Jaasma, L., Bionda, T., Wijnen, J.G.  (2016). Bonobos (Pan Paniscus) show an attentional bias towards conspecifics’ emotions. Proceedings of the National Academy of Sciences,113(14), 3761–3766. I = 9.674. Download.

Kret, M.E., Tomonaga, M. & Matsuzawa, T. (2014). Within-species pupil-synchronization. A comparative study in humans and chimpanzees. PlosOne9(8). I = 3.730. Download.

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