1. People do not feel guilty about exploiting machines
de Melo, C., Marsella, S., & Gratch, J.
ACM Transactions on Computer-Human Interaction, 23, , 2016
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Guilt and envy play an important role in social interaction. Guilt occurs when individuals cause harm to others or break social norms. Envy occurs when individuals compare themselves unfavorably to others and desire to benefit from the others advantage. In both cases, these emotions motivate people to act and change the status quo: following guilt, people try to make amends for the perceived transgression and, following envy, people try to harm envied others. In this paper, we present two experiments that study participants' experience of guilt and envy when engaging in social decision making with machines and humans. The results showed that, though experiencing the same level of envy, people felt considerably less guilt with machines than with humans. These effects occurred both with subjective and behavioral measures of guilt and envy, and in three different economic games: public goods, ultimatum, and dictator game. This poses an important challenge for human-computer interaction because, as shown here, it leads people to systematically exploit machines, when compared to humans. We discuss theoretical and practical implications for the design of human-machine interaction systems that hope to achieve the kind of efficiency cooperation, fairness, reciprocity, etc. we see in human-human interaction.

2. Physiological evidence for a dual process model of the social effects of emotion in computers.
Choi, A., de Melo, C., Khooshabeh, P., Woontack, W., & Gratch, J.
International Journal of Human-Computer Studies, 74, 41-53, 2015
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There has been recent interest on the impact of emotional expressions of computers on people's decision making. However, despite a growing body of empirical work, the mechanism underlying such effects is still not clearly understood. To address this issue the paper explores two kinds of processes studied by emotion theorists in human-human interaction: inferential processes, whereby people retrieve information from emotion expressions about other's beliefs, desires, and intentions; affective processes, whereby emotion expressions evoke emotions in others, which then influence their decisions. To tease apart these two processes as they occur in human-computer interaction, we looked at physiological measures (electrodermal activity and heart rate deceleration). We present two experiments where participants engaged in social dilemmas with embodied agents that expressed emotion. Our results show, first, that people's decisions were influenced by affective and cognitive processes and, according to the prevailing process, people behaved differently and formed contrasting subjective ratings of the agents; second we show that an individual trait known as electrodermal lability, which measures people's physiological sensitivity, predicted the extent to which affective or inferential processes dominated the interaction. We discuss implications for the design of embodied agents and decision making systems that use emotion expression to enhance interaction between humans and computers.

3. Reading people's minds from emotion expressions in interdependent decision making.
de Melo, C., Carnevale, P., Read, S., & Gratch, J.
Journal of Personality and Social Psychology, 106(1), 73-88, 2014
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How do people make inferences about other people's minds from their emotion displays? The ability to infer others' beliefs, desires and intentions from their facial expressions should be especially important in interdependent decision making when people make decisions from beliefs about the others' intention to cooperate. Five experiments tested the general proposition that people follow principles of appraisal when making inferences from emotion displays, in context. Experiment 1 found that the same emotion display produced opposite effects depending on context: when the other was competitive, a smile on the other's face evoked a more negative response than when the other was cooperative. Experiment 2 found that the essential information from emotion displays was derived from appraisals (e.g., is the current state-of-affairs conducive to my goals? Who is to blame for it?); facial displays of emotion had the same impact on people's decision making as textual expressions of the corresponding appraisals. Experiments 3, 4 and 5 used multiple mediation analyses and a causal-chain design: Results supported the proposition that beliefs about others' appraisals mediate the effects of emotion displays on expectations about others' intentions. We suggest a model based on appraisal theories of emotion that posits an inferential mechanism whereby people retrieve, from emotion expressions, information about others' appraisals, which then lead to inferences about others' mental states. This work has implications for the design of algorithms that drive agent behavior in human-agent strategic interaction, an emerging domain at the interface of computer science and social psychology.

4. Humans vs. computers: Impact of emotion expressions on people's decision making.
de Melo, C., Carnevale, P., & Gratch, J.
IEEE Transactions on Affective Computing, 6(2), 127-136, 2014
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Recent research in perception and theory of mind reveals that people show different behavior and lower activation of brain regions associated with mentalizing (i.e., the inference of other's mental states) when engaged in decision making with computers, when compared to humans. These findings are important for affective computing because they suggest people's decisions might be influenced differently according to whether they believe emotional expressions shown in computers are being generated by algorithms or humans. To test this, we had people engage in a social dilemma (Experiment 1) or negotiation (Experiment 2) with virtual humans that were either perceived to be agents (i.e., controlled by computers) or avatars (i.e., controlled by humans). The results showed that such perceptions have a deep impact on people's decisions: in Experiment 1, people cooperated more with virtual humans that showed cooperative facial displays (e.g., joy after mutual cooperation) than competitive displays (e.g., joy when the participant was exploited) but, the effect was stronger with avatars (d = .601) than with agents (d = .360); in Experiment 2, people conceded more to angry than neutral virtual humans but, again, the effect was much stronger with avatars (d = 1.162) than with agents (d = .066). Participants also showed less anger towards avatars and formed more positive impressions of avatars when compared to agents.

5. The impact of emotion displays in embodied agents on emergence of cooperation with people.
de Melo, C., Carnevale, P., Gratch, J.
Presence: Teleoperators and Virtual Environments Journal, 20(5), 449-465, 2012
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Acknowledging the social functions of emotion in people, there has been growing interest in the interpersonal effect of emotion on cooperation in social dilemmas. This article explores whether and how facial displays of emotion in embodied agents impact cooperation with human users. The article describes an experiment where participants play the iterated prisoner's dilemma against two different agents that play the same strategy (tit-for-tat), but communicate different goal orientations (cooperative vs. individualistic) through their patterns of facial displays. The results show that participants are sensitive to differences in the emotion displays and cooperate significantly more with the cooperative agent. The results also reveal that cooperation rates are only significantly different when people play first with the individualistic agent. This is in line with the well-known black-hat/white-hat effect from the negotiation literature. However, this study emphasizes that people can discern a cooperator (white-hat) from a non-cooperator (black-hat) based only on emotion displays. We propose that people are able to identify the cooperator by inferring from the emotion displays, the agent's goals. We refer to this as reverse appraisal, as it reverses the usual process in which appraising relevant events with respect to one's goals leads to specific emotion displays. We discuss implications for designing human-computer interfaces and understanding human-human interaction.

6. Affective engagement to emotional facial expressions of embodied social agents in a decision-making game.
Choi, A., de Melo, C., Woo, W., & Gratch, J.
Computer Animation and Virtual Worlds, 23(3-4), 331-342, 2012
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Previous research illustrates that people can be influenced by the emotional displays of computer-generated agents. What is less clear is if these influences arise from cognitive or affective process (i.e., do people use agent displays as information or do they provoke user emotions). To unpack these processes, we examine the decisions and physiological reactions of participants (heart rate and electrodermal activity) when engaged in a decision task (prisoner's dilemma game) with emotionally expressive agents. Our results replicate findings that people's decisions are influenced by such emotional displays, but these influences differ depending on the extent to which these displays provoke an affective response. Specifically, we show that an individual difference known aselectrodermal labilitypredicts the extent to whether people will engage affectively or strategically with such agents, thereby better predicting their decisions. We discuss implications for designing agent facial expressions to enhance social interaction between humans and agents.

7. The influence of autonomic signals on perception of emotions in embodied agents.
de Melo, C., Kenny, P., & Gratch, J.
Applied Artificial Intelligence, 24(6), 494-509, 2010
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Specific patterns of autonomic activity have been reported when people experience emotions. Typical autonomic signals that change with emotion are wrinkles, blushing, sweating, tearing, and respiration. This article explores whether these signals can also influence the perception of emotion in embodied agents. The article first reviews the literature on specific autonomic signal patterns associated with certain affective states. Next, it proceeds to describe a real-time model for wrinkles, blushing, sweating, tearing, and respiration that is capable of implementing those patterns. Two studies are then described. In the first, subjects compare surprise, sadness, anger, shame, pride, and fear expressed in an agent with or without blushing, wrinkles, sweating, or tears. In the second, subjects compare excitement, relaxation, focus, pain, relief, boredom, anger, fear, panic, disgust, surprise, startle, sadness, and joy expressed in an agent with or without typical respiration patterns. The first study shows a statistically significant positive effect on perception of surprise, sadness, anger, shame, and fear. The second study shows a statistically significant positive effect on perception of excitement, pain, relief, boredom, anger, fear, panic, disgust, and startle. The relevance of these results to artificial intelligence and intelligent virtual agents is discussed.

8. Real-time expression of affect through respiration.
de Melo, C., Kenny, P., & Gratch, J.
Computer Animation and Virtual Worlds, 21(3-4), 225-234, 2010
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Affect has been shown to influence respiration in people. This paper takes this insight and proposes a real-time model to express affect through respiration in virtual humans. Fourteen affective states are explored: excitement, relaxation, focus, pain, relief, boredom, anger, fear, panic, disgust, surprise, startle, sadness, and joy. Specific respiratory patterns are described from the literature for each of these affective states. Then, a real-time model of respiration is proposed that uses morphing to animate breathing and provides parameters to control respiration rate, respiration depth and the respiration cycle curve. These parameters are used to implement the respiratory patterns. Finally, a within-subjects study is described where subjects are asked to classify videos of the virtual human expressing each affective state with or without the specific respiratory patterns. The study was presented to 41 subjects and the results show that the model improved perception of excitement, pain, relief, boredom, anger, fear, panic, disgust, and startle.

9. Multimodal expression in virtual humans.
de Melo, C., & Paiva, A.
Computer Animation and Virtual Worlds, 17(3-4), 1-10, 2006
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This work proposes a real-time virtual human multimodal expression model. Five modalities explore the affordances of the body: deterministic, non-deterministic, gesticulation, facial, and vocal expression. Deterministic expression is keyframe body animation. Non-deterministic expression is robotics-based procedural body animation. Vocal expression is voice synthesis, through Festival, and parameterization, through SABLE. Facial expression is lip-synch and emotion expression through a parametric muscle-based face model. Inspired by psycholinguistics, gesticulation expression is unconventional, idiosyncratic, and unconscious hand gestures animation described as sequences of Portuguese Sign Language hand shapes, positions and orientations. Inspired by the arts, one modality goes beyond the body to explore the affordances of the environment and express emotions through camera, lights, and music. To control multimodal expression, this work proposes a high-level integrated synchronized markup languageexpressive markup language. Finally, three studies, involving a total of 197 subjects, evaluated the model in storytelling contexts and produced promising results.

Last updated: February 19th, 2017