Introduction to Social Neuroscience

Stephanie Cacioppo, John T. Cacioppo

• Publisher: Princeton University Press
• Year: 2020
• ISBN:
  9780691189178
• Topic: Biological Sciences
• Subtopic: Neuroscience
• Index: Biological Sciences

A textbook that lays down the foundational principles for understanding social neuroscience Humans, like many other animals, are a highly social species. But how do our biological systems implement social behaviors, and how do these processes shape the brain and biology? Spanning multiple disciplines, Introduction to Social Neuroscience seeks to engage students and scholars alike in exploring the effects of the brain's perceived connections with others. This wide-ranging textbook provides a quintessential foundation for comprehending the psychological, neural, hormonal, cellular, and genomic mechanisms underlying such varied social processes as loneliness, empathy, theory-of-mind, trust, and cooperation.Stephanie and John Cacioppo posit that our brain is our main social organ. They show how the same objective relationship can be perceived as friendly or threatening depending on the mental states of the individuals involved in that relationship. They present exercises and evidence-based findings readers can put into practice to better understand the neural roots of the social brain and the cognitive and health implications of a dysfunctional social brain. This textbook's distinctive features include the integration of human and animal studies, clinical cases from medicine, multilevel analyses of topics from genes to societies, and a variety of methodologies.Unveiling new facets to the study of the social brain's anatomy and function, Introduction to Social Neuroscience widens the scientific lens on human interaction in society.

• The first textbook on social neuroscience intended for advanced undergraduates and graduate students
• Chapters address the psychological, neural, hormonal, cellular, and genomic mechanisms underlying the brain's perceived connections with others
• Materials integrate human and animal studies, clinical cases, multilevel analyses, and multiple disciplines

1

AN INTRODUCTION TO SOCIAL NEUROSCIENCE

The brain is the most complex organ in the known universe, and specifying the neural mechanisms underlying social structures and interactions has become one of the grand challenges for the neurosciences to address in the twenty-first century.¹ Such an endeavor is both exciting and daunting because it necessitates the integration of theories, methods, and data across levels of organization from multiple disciplines and social species. To meet this challenge, the field of social neuroscience has grown dramatically as an interdisciplinary science. Our goal in this book is to introduce you to this field.

1.1 The Evolution of Social Behaviors

Our journey begins with the question of how did social behaviors evolve? Social behaviors can be classified according to the fitness consequences for the actor and its social partners. Four types of social behaviors that are found in species ranging from bacteria to humans are:² (1) mutual benefit—a social behavior that benefits all involved in the interaction; (2) selfishness—a behavior that benefits the actor at the expense of the other(s) involved in the interaction; (3) altruism—a behavior that is costly for the actor but that benefits other(s); and (4) spite—a behavior that is costly for the actor and the other(s).²

Social behaviors that fall under the category of mutual benefit or selfishness have direct effects on the fitness of the actor and, therefore, are favored through natural selection. Social behaviors that fall under the category of altruism and spite reduce the fitness of the actor, but the same evolutionary processes can select for these behaviors when certain conditions set forth in Hamilton’s rule are met.

According to Hamilton’s rule, altruistic behaviors are favored when the cost to the actor is smaller than the product of the benefit to the other(s) and the relatedness of the other(s) to the actor, where genetic relatedness describes the genetic similarity between two individuals, relative to a reference population. For instance, positive relatedness means that two individuals share more genes than average, and negative relatedness means two individuals share fewer genes than average. By the same logic, spiteful behaviors are favored when the cost to the actor is smaller than the product of the cost to the other(s) and the negative relatedness of the other(s) to the actor, or when the mutually costly behaviors represent a cost to the actor that is smaller than the product of the benefit for a third party and the relatedness of the third party to the actor.^2,3

It also follows from Hamilton’s rule that genetic relatedness, which can be signaled by factors such as kin recognition, may play a larger role in the evolution of social behaviors that fall under the categories of altruism or spite than in the evolution of social behaviors that fall under the categories of mutual benefit and selfishness.

The evolutionary principles favoring social behaviors are the same across species, and social behaviors evolved long before the appearance of humans. The human brain shares many design features with those of other organisms, and both comparative studies and animal models play an important role in revealing the secrets of brain function. The human brain also differs from that of other species. Apparently uniquely, the human brain contemplates the history of the earth, the reach of the universe, the origin of our species, the genetic blueprint of life, and the physical basis of our own unique mental existence.⁴ Two observations that arise repeatedly in our journey through social neuroscience are that (1) there are conserved neural, hormonal, cellular, and molecular mechanisms underlying social behavior; and (2) social connections (e.g., kinship), social complexity (e.g., possible interaction partners), and social and cultural learning are driving forces behind the evolution of the remarkable capacities of the human brain.

1.2 The Social Brain of the Desert Locust

Social species are so characterized because they interact frequently with members of their own species (or what is termed “conspecifics”) to form structures (i.e., patterns of interaction such as pair bonds, mother-infant attachments, and teams) that extend beyond the individual. Frequent interactions with conspecifics introduces complexities, demands, challenges, dangers, opportunities, and benefits not faced by nonsocial species. As a result, not only does the brain underlie social processes and structures, but social structures and processes can influence brain function and structure. These influences are generally thought to occur over generations through evolutionary processes such as natural selection. However, the influence of the social environment on brain structures and function can also be seen within a lifetime.

At any single moment in time, an individual member of a social species may vary in terms of its position along a continuum of social integration (salubrious social connections and bonds) to social isolation (e.g., exclusion, neglect). Where an organism falls along this continuum can be studied longitudinally in natural settings or manipulated experimentally in the laboratory to investigate the causal effects of the social context. Research has shown that where an organism falls along this continuum can influence brain structures and functions.⁵ We survey a number of such influences in chapter 2 but consider the desert locust (Schistocerca gregaria) as a case in point (figure 1.1).

The desert locust is found in Africa, Asia, and the Middle East. It is a voracious insect, eating its weight in fruit, leaves, seeds, flowers, stems, bark, and shoots. At 2 grams, the daily consumption of an individual insect is insignificant. However, in their gregarious state, desert locusts form swarms of fast flying insects numbering as many as 50 billion and consuming up to 200,000 tons of food per day. Swarms of desert locust have ravaged crops and spawned famine for centuries.

What makes the desert locust of special interest here is that it can switch back and forth between an asocial state and a social state. The asocial state is the more typical condition, during which period the locust generally avoids conspecifics. Under specifiable conditions (e.g., stimulation indicative of swarming), the locusts transform from a solitary state to a gregarious (social) state, at which point the brains of these locusts grow approximately 30% larger presumably to accommodate the additional information-processing demands of their now more complicated social environment (figure 1.1).⁶

The deprivation of these social connections leads to a return to the asocial state, along with a consequent reduction in brain volume. Importantly, the brains of these locusts do not grow generally, but rather the growth is in brain regions that are particularly important in the swarming phase (box 1.1).

Social processes in humans were once thought to have been incidental to human learning and cognition. However, there is growing evidence that this is not the case, and instead that the social complexities and demands of primate species have contributed to the evolution of the neocortex and to various aspects of human cognition.^7,8 For instance, cross-species comparisons have revealed that the evolution of large and metabolically expensive brains is more closely associated with social than ecological complexity.⁹ Moreover, although human toddlers and chimpanzees have similar cognitive skills for engaging and interacting in the physical world, toddlers show more sophisticated cognitive skills than chimpanzees for engaging the social world.¹⁰ We further address this topic in chapter 3.

FIGURE 1.1. A. Top panel. The desert locust (Schistocera gregaria). iStock. com/MaYcal. B. Bottom panel. Half-brains of a solitarious locust (left) and gregarious locust (right) in frontal view to the same scale (scale bar, 1 mm). The locusts were of near-identical body size. This image shows the visual neuropiles of the optic lobe that include the medulla (lime), the lamina (cyan), and the lobula (red). Adapted from Ott and Rogers.6 From Fig. 1 of Swidbert R. Ott and Stephen M. Rogers. Gregarious desert locusts have substantially larger brains with altered proportions compared with the solitarious phase. Proceedings of the Royal Society B: Biological Sciences http://doi.org/10.1098/rspb.2010.0694.

BOX 1.1. Use It or Lose It

Not the entire brain increases in size when submitted to complex social interactions. Because the brain is energetically expensive, it has been posited that specific brain regions should enlarge only when needed to meet functional demands.46 In other words, the size of a neural region is related to its functional significance. If social connections/isolation follows the rule of “use it or lose it,” regional neuroanatomical adjustments should occur contingent on the demands of social versus isolated living conditions. Consistent with this reasoning, experimental studies of social isolation or solitary states on brain size indicate that the effects are not uniform across the brain but instead are most evident in brain regions that reflect differences in the functional demands of solitary versus social living for that particular species. For instance, the gregarious locust has a larger midbrain to optic lobe ratio, and within both the visual and olfactory systems higher multimodal integration centers are disproportionately larger than the primary sensory neuropils.6,48 The central complex, an important multimodal sensory and sensorimotor integration center, is also considerably larger in gregarious locusts. Despite the solitary desert locust having a smaller brain overall, the solitary locust has disproportionally large primary visual and olfactory neuropils, putatively due to the increased individual predation risk and the need for the solitary locust to detect visual stimuli at a greater distance.48 Similar reductions in regional brain size in socially isolated animals have been found in other animals, including Drosophila melanogaster,49 several species of honeybees,(see 5 for review) and mammals.27 For instance, Technau49 showed that socially isolated adult female wild-type Kapelle Drosophila melanogaster have fewer mushroom body fibers than do members of a control group—the mushroom bodies in D. m...