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Miranda Smith, Karis Gillen, Brandon Filllingim, Jack Roskiewicz

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What are the neural and psychological mechanisms involved in trust behaviors in primates?

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The dictionary describes trust as the "assured reliance on the character, ability, strength, or truth of someone or something." It correlates to someone being dependent or hopeful on someone else's future actions (Merriam-Webster, 2024).

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When primates trust one another, their relationships often follow a reciprocal pattern marked by cooperation, care, and mutual benefit. One of the clearest signs of this trust is social grooming. More than just a hygienic act, grooming is a deeply social behavior—it soothes, reinforces bonds, and communicates safety and acceptance (Engelmann et al., 2015).

Food sharing is another strong indicator of trust (Engelmann et al., 2015). In many primate species, food is a valuable and sometimes scarce resource, so sharing it comes at a cost. Yet trusted partners—whether friends, kin, or coalition allies—are more likely to share food, especially after cooperative acts like hunting or foraging. This kind of sharing isn’t random; it’s strategic and emotional, based on a history of positive interactions and a sense of mutual reliability.

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Chimpanzees Grooming Each Other

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In order to cooperate with others, animals have to trust them and their actions. Cooperation can only work if everyone is selfless, which can only happen when trust is involved (Lecture 329).

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The Prisoner's Dilemma is the most well-known game theory scenario that analyzes the behavior of two rational, self-interested individuals. In this scenario, these individuals are prisoners, and both have two options: confess or cooperate (stay silent). If both individuals cooperate, they will both receive a shorter sentence. However, if either tells on the other, or both tell on each other, they will receive much higher sentences. This game serves to test the cooperation and trustworthiness of individuals put in this situation. It shows the conflict of acting out of self-interest vs. for the benefit of the group. There are plenty of real-world settings where this dilemma can occur. Primates are constantly put in this position: having to determine whether to share or withhold resources (Lecture 329).

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Measuring Trust Using the Prisoner's Dilemma

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Measuring Trust Using the Prisoner's Dilemma

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Engelmenn and colleagues (2015) created a modified version of the Prisoner's Dilemma to measure trusting behavior in chimpanzees. In this experiment, chimpanzees would choose between pulling the no-trust rope, which would immediately offer them a low-quality prize, or they could pull the trust rope, which would send two compartments of high-quality prizes to a chimpanzee in another room. The partner chimpanzee would then decide whether to send a high-quality prize back to the first chimpanzee or not. If the first chimpanzee sent the high-quality prize to his partner, this was classified as trustworthy behavior, if they took the smaller prize, this was classified as untrustworthy behavior.

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In one study, researchers looked at how primates evaluate social information. Social information is required when a primate evaluates if they should trust someone else. They measured neuronal activity during activities when the monkey was alone or had to work with a partner.

The task included a monkey having to keep their eyes fixated on a visual stimulus and put their hand on a lever that would give them a reward. If they clicked the lever too early or did not keep their eyes on the fixation point, they were not rewarded. In the task with a partner, both animals had to keep their fixation on the visual stimulus, but only one of them had to pull the lever (Azzi et al., 2012).

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Experimenting with Trust and Reciprocity

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Experimenting with Trust and Reciprocity

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Schmelz et al. (2020) had studied prosocial behaviors in chimpanzees. Their research question was, do chimpanzees show a preference for acting prosocially toward individuals who have directly helped them, or toward those who exhibit prosocial behavior more generally toward others?

This experiment was conducted with fifteen chimpanzees from a sanctuary in Kenya, where each chimp had to choose between pulling two different ropes. Pulling the first rope gave the chimp immediate access to low-quality food, and the partner was not involved. Pulling the second rope sent a vehicle to the chimp’s partner. The vehicle had two compartments, but the partner could only eat from one. After eating, the partner could choose to either send the vehicle back (showing trust) or keep it and not send it back (showing no trust).

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The primary goal of this study was to evaluate the role of the amygdala in Primate social function. The authors also wanted to test the effect of administered oxytocin on prosociality. They recorded extracellular activity from 150 BLA (Basolateral Amygdala) neurons on Rhesus Macaques. These monkeys were tasked with making juice decisions for themselves, another monkey present, both animals, or neither.

The next part of their study was administering oxytocin or saline (control) into these monkeys to then rerun the previous trials on making juice decisions. They administered these into the BLA and dlPFC (another control) of the monkeys. literature on the role of the BLA and oxytocin's effect on prosocial behavior and strengthen our understanding of them (Chang et al., 2015).

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Results from prisoner's dilemma-like experiments show that primates use cognitive processes that influence who they decide to trust and when. Most commonly, chimpanzees demonstrate impressive learning capabilities, remembering individuals they have interacted with in the past, and using their experiences to guide their future decision-making. This can be seen in Figure 1., primates showed increased trust behavior in partners who would reciprocate the sharing behavior, and decreased trust in partners who would betray them. Additionally, the chimpanzees would show strong emotional reactions to betrayal, indicating a potentially strong emotional component to decision making and trust (Engelmann et. al., 2015).

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The results from Schmelz rope experiment showed that chimpanzees were more likely to cooperate with the chimp who had previously helped them directly (the direct condition). They found that they were open to trusting others at first, but if someone didn’t prove trustworthy, they learned and changed their behavior (Schmelz et al., 2020).

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To study neurological mechanisms, researchers can use single-unit recording to evaluate neuronal activity. They insert a microelectrode into the target brain region, and it records the electrical activity of a single neuron. Researchers can see if the neuron's activity increases or decreases when the animal is completing a specific task. This allows them to know the function of that neuron and the function of the region as a whole (Azzi et al., 2012). Both studies mentioned used this method.

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Oxytocin is a natural hormone created by the hypothalamus. Oxytocin deals with behavior and social interactions such as trust, recognition, sexual arousal, and romantic attachment. It is also known as the love hormone (Olff et al., 2013).

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Located in the temporal region of the brain, the amygdala's main functions are processing emotions, memories, and social cues. A large regulator of fear and anxiety, the amygdala is also responsible for initiating the "fight or flight" response (Abuhasan et al., 2023).

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Researchers found that mean firing rates for the Basolateral Amygdala (BLA) were positively associated with reward value across self, both, and other conditions, but not for the neither condition. They also administered oxytocin or saline (control) into these monkeys to test the previous literature on oxytocin's effect on improving prosocial behavior. They found that administering Oxytocin to the BLA improved prosocial behavior in these monkeys. These results support previous literature on the role of the BLA and oxytocin's effect on prosocial behavior and strengthen our understanding of them (Chang et al., 2015).

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The orbitofrontal cortex (OFC) has a function in cognitive processes and decision-making. Using single-unit recording, researchers noticed that the OFC had high activation when the monkey was working with a partner. They found that the OFC is involved in identifying individuals, identifying rewards, and identifying social dominance. Identifying individuals is a key aspect of trust. In order to continue trusting individuals, one has to remember who they are and their past actions. When it comes to understanding social dominance, they may want to take this into account when trusting an individual. Lower-ranking individuals may be less trustworthy or vice versa (Azzi et al., 2012).

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video:

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AbuHasan, Q., Siddiqui, W., & Reddy, V. (2023, July 17). Neuroanatomy, amygdala. Neuroanatomy, Amygdala. https://www.ncbi.nlm.nih.gov/books/NBK537102/

Azzi, João C. B., et al. “Modulation of Value Representation by Social Context in the Primate Orbitofrontal Cortex.” Proceedings of the National Academy of Sciences, vol. 109, no. 6, 23 Jan. 2012, pp. 2126–2131, https://doi.org/10.1073/pnas.1111715109.

Chang, S. W., Fagan, N. A., Toda, K., Utevsky, A. V., Pearson, J. M., & Platt, M. L. (2015). Neural mechanisms of social decision-making in the primate amygdala. Proceedings of the National Academy of Sciences of the United States of America, 112(52), 16012–16017. https://doi.org/10.1073/pnas.1514761112

Engelmann, J. M., Herrmann, E., & Tomasello, M. (2015). Chimpanzees trust conspecifics to engage in low-cost reciprocity. Proceedings of the Royal Society B: Biological Sciences, 282(1801), 20142803. https://doi.org/10.1098/rspb.2014.2803

Merriam-Webster. (2024). Definition of TRUST. Merriam-Webster.com. https://www.merriam-webster.com/dictionary/trust

Olff, M., Frijling, J. L., Kubzansky, L. D., Bradley, B., Ellenbogen, M. A., Cardoso, C., Bartz, J. A., Yee, J. R., & van Zuiden, M. (2013). The role of oxytocin in social bonding, stress regulation and mental health: An update on the moderating effects of context and interindividual differences. Psychoneuroendocrinology, 38(9), 1883–1894. https://doi.org/10.1016/j.psyneuen.2013.06.019

Schmelz, M., Grueneisen, S., & Tomasello, M. (2020). The psychological mechanisms underlying reciprocal prosociality in chimpanzees (Pan troglodytes). Journal of Comparative Psychology, 134(2), 149–157. https://doi.org/10.1037/com0000200

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Modulation of Value Representation by Social Context in the Primate Orbitofrontal Cortex

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Modulation of Value Representation by Social Context in the Primate Orbitofrontal Cortex

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Contributer: Karis Gillen

The authors wanted answers to the neuronal processing that occurs during social processing, specifically in the Orbitofrontal Cortex (OFC). Researchers did a single-unit recording of a neuron in the OFC in two macaque monkeys. The monkeys were trained to complete a visual discrimination task where they had to view a visual stimulus on a screen and then press a lever to release a reward. The trials were split into a social and a nonsocial block. In the nonsocial block, a reward in the form of water was given if the monkey was active. In the social block, medium-sized water rewards were given to the monkey and/or a designated monkey partner. They also found that certain OFC cells encoded for the identity of the rewarded individual. These neurons did not fire during the nonsocial trials. Indicating they are not factors in motivation. It shows that the OFC has two (or more) types of neurons, one that encodes for the worth of the reward and one that encodes for the identity of who is getting the reward. When the monkey looked at their partner’s face, a subset of OFC neurons fired, indicating that they represent facial identity. Interestingly, these neurons fired more when the monkey was looking at the socially dominant partner. Meaning they may also encode for social dominance identity. Overall, this paper will help my team better understand which parts of the brain are involved in identifying social status and familiar partners to trust. I was left wondering if the OFC plays a larger role in trust.

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Azzi, João C. B., et al. “Modulation of Value Representation by Social Context in the Primate Orbitofrontal Cortex.” Proceedings of the National Academy of Sciences, vol. 109, no. 6, 23 Jan. 2012, pp. 2126–2131, https://doi.org/10.1073/pnas.1111715109. 

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The psychological mechanisms underlying reciprocal prosociality in chimpanzees

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The psychological mechanisms underlying reciprocal prosociality in chimpanzees

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Contributor: Miranda Smith

This study aimed to examine what drives chimpanzees to cooperate with certain individuals, especially in situations where cooperation is costly—meaning they give something up to benefit another. The authors proposed two hypotheses: first, that chimpanzees cooperate with individuals who have acted prosocially toward them in the past, and second, that they cooperate with those who have demonstrated strong cooperation skills in general. The study consisted of two experiments. In Experiment 1, chimpanzees could choose to share food with another chimpanzee who had either acted prosocially toward them (the direct condition) or with a chimpanzee who had shared food with a third-party chimpanzee (the indirect condition). The results showed that chimpanzees were more likely to cooperate with the chimp who had previously helped them directly (the direct condition). In Experiment 2, chimpanzees were given the choice to share food with a chimpanzee who had not performed any prosocial behavior previously. The results from this experiment indicated that chimpanzees cooperated less compared to Experiment 1. Overall, the findings support the idea that chimpanzees engage in reciprocal cooperation driven by social bonding and direct experiences with partners who have helped them in the past. Chimpanzees were more willing to cooperate with individuals who had directly helped them and were less likely to cooperate with individuals who had only helped others, unless they were directly involved in the cooperation.

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Schmelz, M., Grueneisen, S., & Tomasello, M. (2020). The psychological mechanisms underlying reciprocal prosociality in chimpanzees (Pan troglodytes). Journal of Comparative Psychology, 134(2), 149–157. https://doi.org/10.1037/com0000200 

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Neural mechanisms of social decision-making in the primate amygdala

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Neural mechanisms of social decision-making in the primate amygdala

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Contributor: Brandon Fillingim

The primary goal of this study was to evaluate the role of the amygdala in Primate social function. The amygdala has been linked to decision making and social behavior and more importantly, fear. The authors also wanted to test the effect of administered oxytocin on prosociality. They recorded extracellular activity from 150 BLA (Basolateral Amygdala) neurons on Rhesus Macaques. These monkeys were tasked with making juice decisions for themselves, another monkey present, both animals, or neither. The results from the first trials of this experiment showed that when faced with “neither or other” the monkey would provide juice to the other monkey. However, when shown “self or both” the monkey would choose self. The Basolateral region of the Amygdala is the area that the authors are specifically interested in. After running these trials and observing the neuron activity, they found that mean firing rates were positively associated with reward value across self, both, and other conditions, but not for the neither condition. Considering that this region was much more active when making decisions about itself or others, these results further support the BLA’s role in social behavior and decision making. The next part of their study was administering oxytocin or saline (control) into these monkeys to test the previous literature on oxytocin's effect on improving prosocial behavior. They administered these into the BLA and dlPFC (another control) of the monkeys. They found that administered Oxytocin to the BLA improved prosocial behavior in these monkeys. This effect was insignificant when administering oxytocin to the vlPFC. There were no significant effects when each reward context was considered independently. This means that for the significant result of prosocial behavior, it includes “self, other, and both” and shows the oxytocin improved all of these choices, but not particularly one over the other. The findings of this study provide great insight to the role of the BLA and Oxytocin in prosocial behavior in primates. The results show an increase in activation when a Rhesus Macaque participates in behavior that benefits themselves or others. The administration of oxytocin to this region also encourages this behavior, and makes it more likely for prosociality to occur. These results support previous literature on the role of the BLA and oxytocin's effect on prosocial behavior and strengthen our understanding of them.

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Chang, S. W., Fagan, N. A., Toda, K., Utevsky, A. V., Pearson, J. M., & Platt, M. L. (2015). Neural mechanisms of social decision-making in the primate amygdala. Proceedings of the National Academy of Sciences of the United States of America, 112(52), 16012–16017. https://doi.org/10.1073/pnas.1514761112

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Chimpanzees trust conspecifics to engage in low-cost reciprocity

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Chimpanzees trust conspecifics to engage in low-cost reciprocity

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Contributor: Jack Roskiewicz

In this study, researchers presented a group of chimpanzees with a modified version of the human trust game. Chimpanzees would choose between pulling the no-trust rope, which would immediately offer them a low quality prize (a slice of lemon and orange), or they could pull the trust rope, which would send two compartments of high quality prizes (a mix of bananas and apples) to a chimpanzee in another room. The partner chimpanzee would have access to one compartment of high quality food, but the other compartment was locked. The partner chimpanzee would then decide whether to pull the rope to send the prize back to the first chimpanzee who could then eat from the compartment (trustworthy) or not pull the rope(untrustworthy). Researchers found that chimpanzees pulled the trust rope significantly more as compared to the control trials in which there was no monkey in the other room. The partner chimpanzee’s were 32% likely to return the high quality food back to the first chimpanzee. The studies were repeated, and researchers found that the subjects did not trust indiscriminately, but made decisions based on past learned experiences, and would trust reliable partners more (see figure 4. above). Partners in the repeated trials would reciprocate trust 58% of the time, compared to the previous 32%. Additionally, many subjects would show strong emotional reactions to betrayal, which was linked to less trust in future trials. When discussing the psychological mechanisms of trust, researchers found that chimpanzees can spontaneously trust other unrelated chimps with little context. In further trials, chimps would incorporate learned experience into their decision making, and also an emotional component involved in trust related decision making.

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Engelmann, J. M., Herrmann, E., & Tomasello, M. (2015). Chimpanzees trust conspecifics to engage in low-cost reciprocity. Proceedings of the Royal Society B: Biological Sciences, 282(1801), 20142803. https://doi.org/10.1098/rspb.2014.2803 