Sarah Brosnan

Georgia State University, Psychology, Philosophy & Neuroscience, Faculty Member

MD Anderson Cancer Center, Keeling Center for Comparative Medicine and Research, Adjunct

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I study decision-making, particularly in the context of cooperation. I take an explicitly comparative approach, investigating New World and Old World monkeys, great apes, including humans, and other non-primate species, to better understand the distribution of these behaviors across animals and to understand the environmental constraints on cooperation. I bring together the accumulated evidence from different fields to form a better understanding of the evolution of decision making. F or more information, see www.sarah-brosnan.com
Supervisors: Frans de Waal

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Papers by Sarah Brosnan

Sex differences in white matter tracts of capuchin monkey brains

Journal of comparative neurology, May 1, 2023

Nonhuman primates exhibit sexual dimorphism in behavior, suggesting that there could be underlyin... more Nonhuman primates exhibit sexual dimorphism in behavior, suggesting that there could be underlying differences in brain organization and function. Understanding this neuroanatomical variation is critical for enhancing our understanding of the evolution of sex differences in the human brain. Tufted capuchin monkeys (Sapajus [Cebus] apella) represent a phylogenetically diverse taxa of neotropical primates that converge on several behavioral characteristics with humans relevant to social organization, making them an important point of comparison for studying the evolution of sex differences in primates. While anatomical sex differences in gray matter have previously been found in capuchin monkeys, the current study investigates sex differences in white matter tracts. We carried out tract‐based spatial statistical analysis on fractional anisotropy images of tufted capuchin monkeys (15 female, 5 male). We found that females showed significantly higher fractional anisotropy than males in regions of frontal‐parietal white matter in the right cerebral hemisphere. Paralleling earlier findings in gray matter, male and female fractional anisotropy values in these regions were nonoverlapping. This complements prior work pointing toward capuchin sex differences in limbic circuitry and higher‐order visual regions. We propose that these sex differences are related to the distinct socioecological niches occupied by male and female capuchins. Capuchin neuroanatomical sex differences appear to be more pronounced than in humans, which we suggest may relate to human adaptations for prolonged neurodevelopmental trajectories and increased plasticity.

Oxytocin increases during fur‐rubbing regardless of level of social contact in tufted capuchin monkeys

American Journal of Primatology, Mar 26, 2023

Social interactions induce oxytocin release in many social species, suggesting that oxytocin is a... more Social interactions induce oxytocin release in many social species, suggesting that oxytocin is a critical part of social bonding among individuals. However, oxytocin also increases as a result of physical contact and stimulation, making it unclear which features of affiliative behaviors (for instance, social interaction or physical contact with a conspecific) drive the oxytocin increase observed after engaging in these behaviors. We attempted to tease this apart by studying the differential effect of social interaction, visual coordination with a conspecific, and physical stimulation during the fur‐rubbing behavior of tufted capuchin monkeys (Sapajus [Cebus] apella), which often involves social contact with groupmates but is in some cases performed alone. We induced fur‐rubbing by providing onions under three conditions: when capuchins had physical access to their social group and fur‐rubbed in contact with groupmates (social condition), when capuchins were separated from their social group but could still see them fur‐rub (visual coordination), and when capuchins were physically and visually separated from their groupmates (physical stimulation only). We assessed urinary oxytocin in these three conditions and compared them to a control condition in which apples were provided and no fur‐rubbing was observed. Capuchins fur‐rubbed for less time when they could not see their groupmates, but fur‐rubbing increased urinary oxytocin above the control condition in all three fur‐rubbing conditions equally, suggesting that the physical stimulation derived from fur‐rubbing was the most important driver of oxytocin increase. These results support a model in which physical stimulation is an important factor in the relationship between oxytocin and at least some behaviors, suggesting that oxytocin increase alone is not necessarily indicative of a social influence on behavior. Future work is needed to determine the contexts in which social factors do impact oxytocin, and whether the downstream behaviors are the same for socially and nonsocially induced oxytocin release.

Using an evolutionary approach to improve predictive ability in the social sciences: Property, the endowment effect, and law

Evolution and Human Behavior, May 1, 2023

Predicting variation in endowment effect magnitudes

Evolution and Human Behavior, May 1, 2020

Hundreds of studies demonstrate human cognitive biases that are both inconsistent with "rational"... more Hundreds of studies demonstrate human cognitive biases that are both inconsistent with "rational" decisionmaking and puzzlingly patterned. One such bias, the "endowment effect" (also known as "reluctance to trade"), occurs when people instantly value an item they have just acquired at a much higher price than the maximum they would have paid to acquire it. This bias impedes a vast range of real-world transactions, making it important to understand. Prior studies have documented items that do or do not generate endowment effects, and have noted that the effects vary in magnitude. But none has predicted any of the substantial between-item variation in those magnitudes across a large and novel set of items. Working from evolutionary theory, we derived six factors that predicted 52% of the between-item variation in magnitudes for a novel set of 24 items. These results deepen understanding of both the causes of and patterns in endowment effects. More broadly, they suggest that many other cognitive biases may be similarly approached, and potentially linked by a common theoretical framework.

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Validating Urinary Neopterin as a Biomarker of Immune Response in Captive and Wild Capuchin Monkeys

Frontiers in Veterinary Science

Non-invasive health monitoring is advantageous for wild and captive primate populations because i... more Non-invasive health monitoring is advantageous for wild and captive primate populations because it reduces the need for traditional invasive techniques (i.e., anesthetization) that can be stressful and potentially harmful for individuals. The biomarker neopterin is an emerging tool in primatology to measure immune activation and immunosenescence, however, most neopterin studies have focused on catarrhine species with little comparative work examining neopterin and health in platyrrhines. To address this gap, we validated a commercially available enzyme-linked immunosorbent assay (ELISA) to measure urinary neopterin in two types of capuchin monkeys, a wild population of white-faced capuchins (Cebus imitator) and a socially housed captive colony of tufted capuchins (Sapajus apella). We analytically validated methods for measuring urinary neopterin in two capuchin populations and demonstrated that two commonly-used methods to control for urine concentration—creatinine and specific grav...

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Endogenous cortisol correlates with performance under pressure on a working memory task in capuchin monkeys

Scientific Reports

Humans often experience striking performance deficits when their outcomes are determined by their... more Humans often experience striking performance deficits when their outcomes are determined by their own performance, colloquially referred to as “choking under pressure.” Physiological stress responses that have been linked to both choking and thriving are well-conserved in primates, but it is unknown whether other primates experience similar effects of pressure. Understanding whether this occurs and, if so, its physiological correlates, will help clarify the evolution and proximate causes of choking in humans. To address this, we trained capuchin monkeys on a computer game that had clearly denoted high- and low-pressure trials, then tested them on trials with the same signals of high pressure, but no difference in task difficulty. Monkeys significantly varied in whether they performed worse or better on high-pressure testing trials and performance improved as monkeys gained experience with performing under pressure. Baseline levels of cortisol were significantly negatively related to...

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The Evolutionary Roots of Social Comparisons

Social Comparison, Judgment, and Behavior, 2019

A core aspect of human behavior is that people evaluate themselves in comparison to others in the... more A core aspect of human behavior is that people evaluate themselves in comparison to others in their social group. These social comparisons impact how people see themselves, the decisions they make, and the way they behave on a daily basis. Why, however, are people so focused on relative outcomes, rather than looking at their absolute gains? The answer to this question may lie in understanding the evolutionary roots of social decision-making. Like humans, animals regularly make important social decisions. Studies in animal behavior indicate that, both in the lab and in the wild, animals often make these decisions based on relative outcomes, rather than absolutes. This chapter relies on the comparative approach, pulling from studies on animal behavior, anthropology, and comparative psychology, to explore the evolutionary roots of social comparisons, with an emphasis on spanning the gap between controlled laboratory studies and natural contexts.

Behavioral Biology of Capuchin Monkeys

Behavioral Biology of Laboratory Animals, 2021

Sex differences in the brains of capuchin monkeys (Sapajus [Cebus] apella)

Journal of Comparative Neurology, 2020

This study reports an analysis of 20 T1‐weighted magnetic resonance imaging scans from tufted cap... more This study reports an analysis of 20 T1‐weighted magnetic resonance imaging scans from tufted capuchin monkeys (5 male, 15 female). We carried out a data‐driven, whole‐brain volumetric analysis on regional gray matter anatomy using voxel‐based morphometry. This revealed that males showed statistically significant expansion of a region of the hypothalamus, while females showed significant expansion in a distributed set of regions, including the cerebellum, early visual cortex, and higher‐order visual regions spanning occipital and temporal cortex. In order to elucidate the network connectivity of these regions, we employed probabilistic tractography on diffusion tensor imaging data. This showed that the female‐enlarged regions connect with distributed association networks across the brain. Notably, this contrasts with rodent studies, where sex differences are focused in deep, ancestral limbic regions involved in the control of reproductive behavior. Additionally, in our data set, for...

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Chimpanzees Rarely Settle on Consistent Patterns of Play in the Hawk Dove, Assurance, and Prisoner's Dilemma Games, in a Token Exchange Task

Animal Behavior and Cognition, Feb 1, 2019

Games derived from experimental economics can be used to directly compare decision-making behavio... more Games derived from experimental economics can be used to directly compare decision-making behavior across primate species, including humans. For example, the use of coordination games, such as the Assurance game, has shown that a variety of primate species can coordinate; however, the mechanism by which they do so appears to differ across species. Recently, these games have been extended to explore anti-coordination and cooperation in monkeys, with evidence that they play the Nash equilibria in sequential games in these other contexts. In the current paper, we use the same methods to explore chimpanzees' behavior in the Assurance Game; an anti-coordination game, the Hawk Dove game; and a cooperation game with a temptation to defect, the Prisoner's Dilemma game. We predicted that they would consistently play the Nash equilibria, as do the monkeys, and that, as in previous work, the subjects' level of experience with cognitive experiments would impact performance. Surprisingly, few of our pairs consistently played the same outcome (i.e., no statistically significant preferences), although those who did showed evidence consistent with Nash equilibria play, the same pattern seen more consistently in the monkeys. We consider reasons for their inconsistent performance; for instance, perhaps it was due to lack of interest in a task that rewarded them almost every trial no matter what option they chose, although this does not explain why they were inconsistent when the monkeys were not. A second goal of our study was to ascertain the effects of exogenous oxytocin in their decision making in one population. In line with recent work showing complex effects of oxytocin on social behavior, we found no effect on subjects' outcomes. We consider possible explanations for this as well.

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Frans de Waal’s (1982), Chimpanzee Politics

Encyclopedia of Evolutionary Psychological Science, 2021

Frans de Waal

Encyclopedia of Evolutionary Psychological Science, 2021

Capuchin monkeys (Sapajus [Cebus] apella) are more susceptible to contrast than to decoy and social context effects

Theories of optimal decision-making typically assume that animals have consistent preferences amo... more Theories of optimal decision-making typically assume that animals have consistent preferences among options. In reality, economic behaviour in humans and foraging behaviour in some animals is often susceptible to choice-irrelevant factors such as inferior options or conspecifics’ outcomes, but the evidence for primate decision-making is mixed. Unlike previous experiments, we assessed the relative magnitude of three context effects. Using a food preference paradigm, we varied the number of cereal pieces to determine how much a piece of food A was “worth” (50% choice) to each of 13 capuchins. We predicted that monkeys would devalue A in the contrast condition (when a higher-quality but unattainable food was present) and overvalue it in the decoy condition (when a smaller version of A was a third option) and social condition (when A, if unchosen, was given to a partner). Capuchins were 4 times less likely to choose A in the contrast condition, but 2 to 3 times more likely to choose it ...

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The Evolution of Justice

Oxford Scholarship Online, 2018

Where does the human sense of justice come from? An emerging view in the sciences is that the bio... more Where does the human sense of justice come from? An emerging view in the sciences is that the biological foundation of humans’ systems of justice and fairness can be found in the behaviors of other species. This chapter considers what studying other species can and cannot tell us about the evolution of justice and fairness and argues that the foundations of fairness are evolutionarily ancient and based on the need to maintain a relative balance among cooperative partners. Many species appear to recognize when they receive less than a conspecific receives after a cooperative interaction. Humans and a few other species have expanded this to recognizing when they are relatively advantaged. Humans, with their relatively advanced cognition, are the only species to develop a full sense of fairness and possibly because of this, in combination with language, are the only species to have a sense of justice.

Are the roots of human economic systems shared with non-human primates?

Neuroscience & Biobehavioral Reviews, 2020

We review and analyze evidence for an evolutionary rooting of human economic behaviors and organi... more We review and analyze evidence for an evolutionary rooting of human economic behaviors and organization in non-human primates. Rather than focusing on the direct application of economic models that a priori account for animal decision behavior, we adopt an inductive definition of economic behavior in terms of the contribution of individual cognitive capacities to the provision of resources within an exchange structure. We spell out to what extent non-human primates' individual and strategic decision behaviors are shared with humans. We focus on the ability to trade, through barter or token-mediated exchanges, as a landmark of an economic system among members of the same species. It is an open question why only humans have reached a high level of economic sophistication. While primates have many of the necessary cognitive abilities (symbolic and computational) in isolation, one plausible issue we identify is the limits in exerting cognitive control to combine several sources of information. The difference between human and non-human primates' economies might well then be in degree rather than kind.

Considering the Role of Experience in the Formation of Behavioral Biases from a Developmental, Cross-Cultural, and Evolutionary Perspective

Animal Behavior and Cognition, 2019

Humans make countless choices every day that affect their health, safety, and finances. Despite t... more Humans make countless choices every day that affect their health, safety, and finances. Despite the high stakes, decision-making is often irrational from the viewpoint of traditional economics, i.e., the choices made are contrary to existing preferences. This leads to negative consequences for individuals and to inefficiencies in the exchange of goods. The fields of behavioral and experimental economics have made great strides in understanding these sub-optimal patterns of behavior, but we still cannot always predict human decisions. In this paper, we present developmental, cross-cultural, and comparative findings for three irrational tendencies (framing effects, the endowment effect, and inequity aversion) to illustrate two different patterns that experience can play in their formation. This analysis allows us to consider why these tendencies have emerged, what benefits they may bring to the decision-maker, and to propose types of interventions that may combat these tendencies. Throughout, we suggest where this approach of combining comparative and developmental work can address theoretical debates and practical questions within the field of behavioral economics.

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Humans as a model for understanding biological fundamentals

Proceedings of the Royal Society B: Biological Sciences, 2017

How special are humans? This question drives scholarly output across both the sciences and the hu... more How special are humans? This question drives scholarly output across both the sciences and the humanities. Whereas some disciplines, and the humanities in particular, aim at gaining a better understanding of humans per se, most biologists ultimately aim to understand life in general. This raises the question of whether and when humans are acceptable, or even desirable, models of biological fundamentals. Especially for basic biological processes, non-human species are generally accepted as a relevant model to study topics for which studying humans is impractical, impossible, or ethically inadvisable, but the reverse is controversial: are humans 'too unique' to be informative with respect to biological fundamentals relevant to other species? Or are there areas where we share key components, or for which our very uniqueness serves to allow novel explorations? In this special feature, authors from disciplines including biology, psychology, anthropology, neuroscience and philosophy tackle this question. Their overall conclusion is a qualified yes: humans do tell us about biological fundamentals, in some contexts. We hope this special feature will spur a discussion that will lead to a more careful delineation of the similarities and the differences between humans and other species, and how these impact the study of biological fundamentals.

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Urinary oxytocin in capuchin monkeys: Validation and the influence of social behavior

American journal of primatology, Jan 24, 2018

In highly social species, like primates, oxytocin plays an important role in cooperation, and in ... more In highly social species, like primates, oxytocin plays an important role in cooperation, and in the formation and maintenance of social relationships. Despite recent interest in the relationship between oxytocin and social behavior in nonhuman primates, relatively little is known about endogenous oxytocin in social New World Monkeys. In this paper, we investigate the relationship between oxytocin and affiliative behaviors in socially-housed captive capuchin monkeys (Sapajus [Cebus] apella) by first validating methods of analysis of urinary oxytocin in this species and, second, examining the effects of grooming and fur-rubbing behavior on oxytocin concentrations and further affiliative behavior. In the validation, we found that intranasal exogenous oxytocin significantly increased urinary oxytocin 15-60 min post-administration. Oxytocin was also implicated in both grooming and fur-rubbing behaviors. We found that oxytocin concentrations increased after subjects engaged in grooming o...

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