SOCIAL NEUROSCIENCE The Corsini Encyclopedia of Psychology, edited by Irving B. Weiner and W. Edward Craighead. Copyright  2009 John Wiley & Sons, Inc. Social neuroscience is the study of the associations between social and neural levels of organization and the biological mechanisms underlying these associations. Neuroscientists have tended to focus on single organisms, organs, cells, or intracellular processes. Social species create emergent organizations beyond the individual. These emergent structures evolved hand in hand with neural and hormonal mechanisms to support them because the consequent social behaviors helped animals survive, reproduce, and care for offspring sufficiently long that they, too, reproduced. Social neuroscience, therefore, is concerned with how biological systems implement social processes and behavior. It capitalizes on concepts and methods from the neurosciences to inform and refine theories of social psychological processes, and it uses social and behavioral concepts and data to inform and refine theories of neural organization and function. The human brain appears to have evolved to be sub- stantially larger than needed to maintain life. According to the social brain hypothesis, deducing better ways to find food, avoid perils, and navigate territories has adap- tive value for large mammals, but the complexities of these ecological demands pale by comparison with the complexities of social living. The latter include learning by social observation; recognizing the shifting status of friends and foes; anticipating and coordinating efforts between two or more individuals; using language to com- municate, reason, teach, and deceive others; orchestrating relationships, ranging from pair bonds and families to friends, bands, and coalitions; navigating complex social hierarchies, social norms, and cultural developments; sub- jugating self-interests to the interests of the pair bond or social group in exchange for the possibility of long-term benefits; recruiting support to sanction individuals who violate group norms; and doing all this across time frames that stretch from the distant past to multiple possible futures. Accordingly, cross-species comparisons suggest that the evolution of large and metabolically expensive brains is more closely associated with social than ecologi- cal complexity, at least in primates. A biological approach that treats the individual organ- ism as the broadest legitimate unit of organization may illuminate some aspects of development and behavior, but it is unlikely to provide a comprehensive account. Never- theless, from the perspective of many biological scientists during most of the twentieth century, the contributions of the social world to biology and behavior were thought best to be considered later, if at all. Accordingly, social factors were viewed as of minimal interest with respect to the basic development, structure, or processes of the brain. To the extent that social factors were suspected of being relevant, their consideration was thought to so complicate the study of brain and behavior that they were not a pri- ority. The approach of social scientists throughout most of the twentieth century was no less narrowly focused than that of biologists. World wars, a great depression, and civil injustices made it amply clear that social and cul- tural forces were too important to address to await the full explication of cellular and molecular mechanisms. Thus, biological events and processes were routinely ignored. Mounting evidence for the importance of the rela- tionship between social events and biological events has prompted biological scientists, cognitive scientists, and social scientists to collaborate more systematically. The common view is that the understanding of mind and behavior could be enhanced by an integrative analy- sis that encompassed cultures, institutions, collectives, groups, pairing, and individuals as well as brain, cells, proteins, and genes. The growth in this research suggests that linking the neurosciences and social sciences is indeed practical and indicates potential for a common scientific language that can establish bridge principles needed to connect the theoretical terms of these sciences. Social neuroscience is the interdisciplinary scientific field that emerged to bridge these different levels of orga- nization. Examples of the interaction of social and physio- logical domains are legion. So obligatorily gregarious are primates that tactile contact is a stronger determinant of mother–infant attachment than feeding. Early tactile deprivation, even in rodents, can reduce the number of glucocorticoid (a class of stress monitoring and dampening hormones) receptors (binding sites) in the hippocampus and frontal cortex. These changes are persistent, and as a consequence, glucocorticoid stress inhibition is dimin- ished, and stress reactivity is elevated in the pup through- out its life. Intentional action and the observation of intentional action by another individual activate the same neurons—the so-called mirror neuron system—thereby providing a shared neural notation that promotes posi- tive social interactions, synchrony, and communication. Positive social interactions promote the release of oxytocin in the brain, which in turn promotes social recognition and bonding and down-regulates reactivity to stressors. The release of testosterone in nonhuman male primates not only promotes sexual behavior but also the availability of receptive females, in turn, influences testosterone lev- els. Social factors such as perceived isolation are heritable and can affect genetic expression in leukocytes. Although research that falls under the rubric of social neuroscience has a longer tradition, the term social neu- roscience first appeared in a 1992 article calling for an integration of human and animal research traditions to create an interdisciplinary approach to understanding the biological mechanisms that implement social behavior and to using biological concepts and methods to develop and refine theories in the social and behavioral sciences. Research in social neuroscience has grown dramatically