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As reviewed in this chapter, nonhuman animal research has not only produced empirical findings that guide our understanding of how social deprivation and isolation impact developmental outcomes, but also played an important role in the formation of theories such as Bowlby’s attachment theory that have guided a fruitful body of research with humans. Although this chapter has primarily focused on nonhuman animal research, particularly Rhesus macaque. and rodents, there is also an extensive body of work examining the impact of social deprivation and social isolation in humans. For example, one form of social deprivation that is experienced by some children is growing up in institutional settings such as in orphanages. In such settings, children are often deprived of consistent and emotionally responsive caregivers due to factors such as high staff turn overs and rotations, high child to caregiver staff ratios, and insufficient staff training (Smyke et al., 2007). This form of social deprivation has been shown to have long‐lasting negative impact on children’s brain, social, and cognitive development (Hostinar et al., 2012; van Ijzendoorn et al., 2011). These findings are largely consistent with the nonhuman animal research findings reviewed in this chapter.

An important question that nonhuman animal researchers examined is to what extent the adverse impacts of early social deprivation are reversible when social deprivation is terminated. In human research, there is one project that was specifically designed to answer this question: the Bucharest Early Intervention Project (BEIP). The BEIP is a randomized control trial that examines whether children who are removed from institutional care and placed into a high‐quality foster care home show better developmental outcomes compared to those who have remained in the institutions, and equivalent or worse developmental outcomes compared to children who have never been institutionalized. Findings from this project show that children who receive the foster care intervention show better developmental outcomes compared to children who have remained in the institutions in certain areas such as social functioning and intelligence (see Nelson et al., 2019), suggesting that termination of severe social deprivation and transitioning to a responsive caregiving setting help boost the development in certain domains. Moreover, in certain developmental outcomes such as brain electrical activity, findings suggested that children who received the foster care intervention not only showed better developmental outcomes compared to the children who remained in the institutions, but also showed equivalent outcomes compared to those who have never been institutionalized (Debnath et al., 2020), suggesting that some of the adverse effects of institutional care may even be reversible. On the other hand, foster care intervention did not have a detectable positive effect on outcomes such as executive functions, which refers to goal‐directed cognitive skills (Wade et al., 2019). Thus, it might be that the effect of early social deprivation on such outcomes may not be reversible or may be reversible if children are removed within the first six months of development (Colvert et al., 2008). These findings are consistent with that of Harlow’s findings suggesting that the effects of early social deprivation are partially reversible depending on the duration of the social deprivation and timing of terminating social deprivation (Harlow & Suomi, 1971).

Although there is a good amount of human work examining the impact of institutional care or maternal deprivation on child outcomes, few studies have examined whether the impact of social deprivation or recovery from social deprivation depends on the child’s sex. Given the evidence from rodent work suggesting that there may be sex differences in whether or how well rodents recover from social deprivation after resocialization (Arakawa, 2007), it would be important to understand whether the impact of social deprivation or children’s recovery from social deprivation depends on children’s sex. Answering such questions is only possible if studies include large enough samples to be able to compare males and females within the same study. Given the difficulties of conducting research with children who experience social deprivation, recruiting large enough samples is not always possible.

There is also limited research examining what type of social experiences may compensate for maternal or paternal deprivation experienced during different developmental periods. In rodent work, communal rearing with multiple caregivers raising their offspring together have been linked with positive developmental outcomes. Based on this evidence, it would be important to examine which types of communal rearing may promote children’s development. Likewise, it would be important to examine the support of other relatives in child developmental outcomes, as well as whether and how these support mechanisms help children recover from early social deprivation experiences in a more efficient way.

To conclude, research with nonhuman primates and rodents have produced a rich body of work on the impact of social deprivation and social isolation on development. By allowing for greater ability to manipulate the environment, animal research have been useful for identifying the exact mechanisms by which caregivers or social partners may regulate distinct biological and behavioral systems, and explaining how the type of social deprivation (mother, father, peer or community), its timing and duration impacts offspring’s development. In terms of future directions, it will be important to continue to cultivate dialogue between human and nonhuman animal researchers and examine questions related to the impact of early social experiences to determine whether certain findings are generalizable across different species.