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The effects of maternal deprivation on the development of primates is more commonly examined via peer‐rearing paradigms. In this paradigm, monkeys are deprived of having contact with adult monkeys for the first six months of life (Stevens et al., 2009). Instead, in the first month of their lives, monkeys are reared by animal caretakers in a nursery and subsequently housed with three to five peers. This paradigm is considered to reflect the experiences of children living in institutional care more closely than the “partial” or “total isolation” paradigms of Harlow, given that children growing up in institutions are typically deprived of a sensitive adult caregiver with whom they can develop a secure attachment, but they have opportunities to interact with peers.

Research has shown that although peer‐reared primates show attachment to their peers, their attachment is anxious (Stevens et al., 2009). This may be because peers may not be as effective as parents in regulating young monkeys’ fear and distress in the context of unfamiliar or distressing events. Consistent with this finding, peer‐rearing has been shown to have negative impact on primates’ ability to use a social companion to reduce stress as reflected by activation of the HPAA axis, a phenomenon referred to as “social buffering.” Specifically, in three‐year‐old rhesus monkeys, social partners were shown to buffer the stress response of the mother‐reared monkeys; however, the same level of buffering was not observed in the peer‐reared monkeys, suggesting that being deprived of early mother–infant relationship harms primates’ ability to benefit from social buffering (Winslow et al., 2003; for a review, see Hostinar et al., 2014).

Although peer‐reared monkeys showed normal physical and motor development, they developed into more anxious, impulsive, and aggressive individuals and often become more submissive, establishing themselves low in social hierarchy (Suomi, 2008). Their social play was observed to be less frequent and complex. They also showed increased preference for alcohol over nonalcoholic solutions (see Barr et al., 2004). Peer‐reared male monkeys have been shown to have increased levels of health problems in terms of frequency and prevalence compared to those reared by their mothers (Conti et al., 2012).

Peer‐rearing has been shown to lead to greater stress reactivity and disruptions in the hypothalamic‐pituitary adrenal (HPAA) functioning (for a review, Stevens et al., 2009). Specifically, peer‐reared monkeys show higher plasma cortisol (Shannon et al., 1998) and higher basal cortical levels, even after one and a half, and three years of normal social life (Feng et al., 2011). They experience altered neurotransmitter functioning such as lower levels of serotonin‐binding potential (Ichise et al., 2006) and metabolic alterations such as higher rates of whole‐brain glucose metabolism (Doudet et al., 1995).