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 provided an important foundation for studying the importance of social interaction. Indeed, Bowlby’s attachment theory, which was built off this foundation of research, is still deemed to be an important conceptual framework for understanding the role of early social relationships and their absence in both animal and human research.
Nonhuman Primate Models
Why are Nonhuman Primate Models Useful?
Nonhuman primates are the evolutionarily closest animals to humans (Kumar & Hedges, 1998) and, therefore, provide important models to understand the potential effects of deprivations in social experiences on humans. Rhesus monkeys, perhaps the most commonly used nonhuman primates in research, share similarities with humans (Phillips et al., 2014). They share approximately 95% of their genes with humans, and the structure and functional organization of their brains are highly similar to human brains (Stevens et al., 2009). For example, rhesus monkeys’ neocortex constitutes 72% of the brain volume, whereas humans’ neocortex constitutes 80% of the brain volume (Passingham, 2009). In vast contrast, rats’ neocortex comprises only 28% of the rat brain. Consistent with the similarities in neural architecture, primates are also capable of more advanced cognitive skills such as inhibitory control and delay of gratification, and therefore, are good models for examining the effects of early experiences on the development of higher‐level cognition (see Phillips et al., 2014). Similar to humans, nonhuman primates have extended life spans with distinct developmental periods such as infancy, childhood, and adolescence. They spend their early months in close proximity with their caregivers developing an attachment relationship and, then, become gradually more independent from caregivers over time, spending more of their time with peers.
Their social and emotional intelligence also make them good models to understand psychological processes (Phillips et al., 2014). For example, several nonhuman primates including rhesus monkeys and chimpanzees demonstrate complex communication patterns, live in social groups, and demonstrate social learning ability of complex behaviors such washing food before consumption (Stevens et al., 2009). They have the ability to recognize themselves as well as others (see Phillips et al., 2014). Specifically, they are able to discriminate between familiar and unfamiliar faces, recognize individuals’ faces from photos, and understand other conspecifics’ facial expressions (see Phillips et al., 2014).
Nonhuman primates possess relatively stable temperamental and personality dimensions that are comparable to the dimensions studied in humans (Capitanio & Widaman, 2005). For example, chimpanzees have relatively stable traits such as sociability, positive affect, anxiety, and equitability (see Phillips et al., 2014). Likewise, rhesus monkeys’ temperament during infancy has been associated with both the quality (i.e., reciprocated relationships) and quantity (i.e., how much time they spend with others) of their social relationships during juvenile years (Weinstein & Capitanio, 2008).