often described as “social animals” (Batson, 1990). Most animals, however, are social in that they interact with other conspecifics on a regular basis. For example, nonhuman mammals, such as primates and rodents, are both typically reared by their primary caregiver/s in the early stages of their development and continue to interact socially with other conspecifics across their lifetime. Therefore, primate and rodent models offer important opportunities for examining how, why, and when social interactions or their absence matter for healthy development (Brett et al., 2015; Gunnar & Quevedo, 2007). One experimental approach used for answering these questions is depriving animals of specific types of social experiences, such as interactions with mothers or peers, and examining the effects of the social deprivation on development. This work is suitable for answering questions such as: (1) How does maternal, paternal, and/or peer deprivation during specific developmental periods affect development?; (2) Does the developmental timing and duration of social deprivation affect developmental outcomes?; and (3) Can the effects of social deprivation or isolation be reversed by later social interactions or enrichment? Another approach examines whether ordinary variations in animal temperament, particularly fearful responses to unfamiliar social partners, are associated with social isolation and health problems.
In this chapter, we first discuss why it is important to understand the effects of social deprivation and social isolation on humans. Second, we provide a brief review of the theoretical and empirical work on the importance of social relationships in development. Third, we describe the utility of animal models for understanding the potential impact of certain types of social deprivation and isolation experienced by some humans focusing on two main animal models: nonhuman primates and rodents. For each model, we describe the similarities of findings with these species with humans and discuss the unique strengths of research conducted with that population. We then review the nonhuman primate and rodent empirical work on (1) how social deprivation and social isolation as “induced” by experimental manipulations affect development, and (2) how “naturally occurring” variations in temperament, particularly behavioral inhibition, may serve as a risk factor that leads to social isolation and health problems. In conclusion, we discuss the implications of this work for future research directions with humans.
The Importance of Understanding the Effects of Social Deprivation and Social isolation
Understanding the impact of social deprivation and social isolation on healthy human functioning is critical for several reasons. Although most humans are raised by their primary caregivers such as mothers and/or fathers, and continue to form social bonds in later years, there are also wide variations in individuals’ social experiences. For example, around 2.7 million children are estimated to live in institutional care worldwide (Petrowski et al., 2017) and an important proportion of these children are deprived of stable, warm, and responsive caregiving provided by an attachment figure (Nelson et al., 2019). Given that children with a history of institutionalization have vastly varying experiences before, during, and, for some, after institutionalization, assessing the exact quality and quantity of children’s social experiences or the type of deprivation experienced is often challenging. Thus, animal studies are well suited for examining the impact of social deprivation given that they allow for manipulating the exact timing and duration of the deprivation. By allowing for greater “environmental control,” animal models can help manipulate the specific aspects of deprivation, such as deprivation of milk, tactile stimulation or presence/absence of the caregiver, to identify the mechanisms by which social deprivation leads to disruptions in development (Hofer, 1987). Finally, animal models can test whether specific enrichment experiences can reverse specific impacts of social deprivation.
Another important variation in children’s early social experiences concerns the family structure they grow up in. Some children are raised in biparental family units with opportunities to interact with two caregivers on a regular basis, whereas others are raised by single or widowed caregivers without opportunities to interact with a second caregiver. Animal research conducted with monogamous, biparental species allow researchers to examine whether growing up in single‐parent family units and/or experiencing “paternal deprivation” have differential effects on the development of the offspring. Additionally, many individuals experience social isolation during certain periods in their lives. Homeschooled children who also grow up without siblings or peers may be deprived of critical social experiences. Individuals who move to another location, change their schools or workplaces, or lose their loved ones can also experience social isolation. Thus, animal research on the effects of social isolation experienced later in life can inform human studies on whether such social isolation experiences lead to alterations in biology and behavior. Finally, understanding whether and how ordinary variations in these temperaments in animals may be associated with experiences of social isolation and health problems is critical.
Brief History of Theory and Research on the Role of Social Relationships for Development
The importance of the early affective relationships among caregivers and their infants has a long history of emphasis in both human and animal work. For instance, in the 1930s and 1940s, psychoanalytic scholars such as Sigmund Freud and René Spitz highlighted the importance of the early mother infant relationship for healthy development and alerted the public on the negative consequences of separating children from their caregivers and families (Hofer, 1994). In his reports published between 1939 and 1945, Freud brought attention to the devastating effects of family disruptions during World War II on children’s development (Freud & Burlingham, 1974). Spitz’s (1945) work has shown that children of incarcerated women who were allowed to have affective interactions with their mothers showed better developmental outcomes than institutionalized children who were deprived of nurturing social interactions. In animal research, Konrad Lorenz (1935) – regarded as the founder of ethology – demonstrated that ducklings imprint on the first moving object they see shortly after hatching, which was conceptualized as an early forming “emotional bond” between the mother and her offspring (see Lorenz, 1958).
In the 1940s and 1950s, Harry Harlow’s early work on the negative effects of peer‐rearing in rhesus monkeys’ behaviors also highlighted the importance of early interactions between mothers and their offspring. Harlow is most famously known for his studies examining the maternally deprived infant rhesus monkeys’ preference for the surrogate mothers made from cloth over the ones made from wire. His important conclusion from this line of work was that infants’ attachment to their caregivers is not simply due to the fact that caregivers provide food but rather they often are a source for warmth and comfort through the provision of bodily contact (Harlow & Suomi, 1971). Along with the “surrogate mother” studies, Harlow conducted a series of revolutionary studies on the impact of social deprivation and isolation that largely contributed to the development of Bowlby’s attachment theory (Van Der Horst et al., 2008).
John Bowlby, who is considered the founder of attachment theory, integrated the early psychoanalytic perspectives and ethological work on the role of early caregiver–infant relationships into his theory on attachment (Hofer, 1994). Bowlby (1969, 1973, 1982) conceptualized attachment – the special affective bond between a caregiver and his/her child – as an evolutionary‐based motivational system that allows the infant to maintain and restore proximity to the mother to increase his/her chances of survival. Bowlby noted that, when separated from their mothers, infants first “protest” to this separation by crying, searching, and showing distress signs. The evolutionary function of this behavior is to help the caregiver and offspring find each other. Bowlby suggested that, when the separation lasts longer, infants respond with “despair” showing signs of sadness and withdrawn behaviors that serve to conserve energy and avoid danger in the long absence of the attachment figure. Bowlby indicated that an important feature of the attachment relationship is that it is selective, such that the parent and the infant demonstrate attachment behaviors toward one another but not toward others. Both primates and rodents have been shown to respond to short‐term and long‐term separations from their caregivers in similar ways described by Bowlby (for a review on attachment in rhesus monkeys, see Suomi, 2008; for a review on rodent attachment, see Landers & Sullivan, 2012). These seminal studies
