Many of our smiles in everyday life are only posed and signify something very different from joy and happiness

Emotional expressions in human and non-human great apes. Mariska E. Kret et al. Neuroscience & Biobehavioral Reviews, January 25 2020, https://doi.org/10.1016/j.neubiorev.2020.01.027

Highlights
• Emotional expressions are frequently used in all great ape species.
• Compared to apes, humans have evolved communicative faces where all the expressive parts are emphasized.
• Also in great apes species-specific facial characteristics have evolved to enhance communication.
• Smiles have been ritualized to a greater extent in humans than in apes, laughter is similar and seen in comparable contexts.
• Great apes do have voluntary control over some expressions.

Abstract: Humans and great apes are highly social species, and encounter conspecifics throughout their daily lives. During social interactions, they exchange information about their emotional states via expressions through different modalities including the face, body and voice. In this regard, their capacity to express emotions, intentionally or unintentionally, is crucial for them to successfully navigate their social worlds and to bond with group members. Darwin (1872) stressed similarities in how humans and other animals express their emotions, particularly with the great apes. Here, we show that emotional expressions have many conserved, yet also a number of divergent features. Some theorists consider emotional expressions as direct expressions of internal states, implying that they are involuntary, cannot be controlled and are inherently honest. Others see them as more intentional and/ or as indicators of the actor’s future behavior. After reviewing the human and ape literature, we establish an integrative, evolutionary perspective and provide evidence showing that these different viewpoints are not mutually exclusive. Recent insights indicate that, in both apes and humans, some emotional expressions can be controlled or regulated voluntarily, including in the presence of audiences, suggesting modulation by cognitive processes. However, even non-intentional expressions such as pupil dilation can nevertheless inform others and influence future behavior. In sum, while showing deep evolutionary homologies across closely related species, emotional expressions show relevant species variation.

Keywords: emotional expressionsgreat apescomparative psychologycognitive controlevolution