Central to Boyer's model is the claim that human ownership psychology results from the interaction of two distinct cognitive systems, one for the competitive acquisition of resources and one for cooperative interactions. He fleetingly notes that ownership intuitions may be “specific to humans” (target article, sect. 2.1.2.) and that possession psychology (based on competitive resource acquisition) is “phylogenetically much older than cooperation behaviors” (target article, sect. 12.2). Boyer does not elaborate much on these points, likely, because he is primarily concerned with explaining human ownership psychology. But these claims could be evaluated by looking at relevant studies with non-human primates, who are – phylogenetically speaking – humans’ closest living relatives.

Let us start by looking at whether there is evidence in non-human primates for Boyer's possession psychology. There are two components to consider here: (1) Whether non-human primates value their possessions, and (2) whether they respect others’ possessions. First, research on the endowment effect (i.e., that one values things in one's possession more than things one does not possess) has shown that capuchin monkeys and all four great ape species value food in their possession (Brosnan et al., Reference Brosnan, Jones, Lambeth, Mareno, Richardson and Schapiro2007; Kanngiesser, Santos, Hood, & Call, Reference Kanngiesser, Santos, Hood and Call2011; Lakshminaryanan, Chen, & Santos, Reference Lakshminaryanan, Chen and Santos2008). They also value tools that can be immediately used to acquire food (Brosnan, Jones, Gardner, Lambeth, & Schapiro, Reference Brosnan, Jones, Gardner, Lambeth and Schapiro2012). Yet great apes do not show endowment effects for tools when there is a short delay before food can be retrieved (Kanngiesser et al., Reference Kanngiesser, Santos, Hood and Call2011), and they are also willing to give up toys in their possession (Brosnan et al., Reference Brosnan, Jones, Lambeth, Mareno, Richardson and Schapiro2007; Drayton, Brosnan, Carrigan, & Stoinski, Reference Drayton, Brosnan, Carrigan and Stoinski2013; Flemming, Jones, Mayo, Stoinski, & Brosnan, Reference Flemming, Jones, Mayo, Stoinski and Brosnan2012). Non-human primates thus value things in their possession if they are of immediate use (like food), but do not exhibit the same range of possessive behaviors that we commonly find in humans. Second, respect for others’ food possession has been studied in non-human primates for both conspecific and human competitors. Sigg and Falett (Reference Sigg and Falett1985) found that dominant hamadryas baboons respected subordinate's food possession in male–male pairs, probably to avoid risk of injury during take-over events (e.g., male baboons have large canines). They observed more frequent take-over events in female–female pairs, and these events were associated with larger differences in rank. Studies with long-tailed macaques showed that subordinates were more likely to keep mobile than fixed food sources because they were able to transport mobile possessions and dominant individuals rarely gave chase (Kummer & Cords, Reference Kummer and Cords1991). Moreover, when confronted with a human competitor, free-ranging rhesus monkeys avoided food that was physically connected to the human by a rope (Russ, Comins, Smith, & Hauser, Reference Russ, Comins, Smith and Hauser2010). These findings suggest that non-human primates use physical proximity and control as possession cues, and that their respect for possession is primarily based on risk of injury, rank order, or opportunities to escape with the food. All in all, the current evidence for non-human primates is compatible with Boyer's possession psychology.

What distinguishes ownership psychology from possession psychology in Boyer's model is the addition of cooperative expectations. Is there evidence for ownership psychology in non-human primates? One way to tackle this question would be to focus on whether relatedness between individuals affects their respect for others’ possessions, as one can reasonably assume that non-human primates have more cooperative inclinations toward kin than non-kin. However, Kummer and Cords (Reference Kummer and Cords1991) found that relatedness had no effect on long-tailed macaques’ respect for food possession. Another avenue would be to create test situations that remove risk of injury (e.g., by preventing direct interactions between individuals) and prevent direct physical control of resources. In a recent study, chimpanzees were in separate rooms with an apparatus placed between them: Each chimpanzee pushed food rewards (wrapped in differently colored paper) toward the middle of the apparatus where they dropped onto trays; individuals could then pull the trays to their side to retrieve the rewards (Kanngiesser, Rossano, Frickel, Tomm, & Tomasello, Reference Kanngiesser, Rossano, Frickel, Tomm and Tomasello2020). When both individuals had simultaneous access to the trays, they retrieved food irrespective of who had worked for it. When one individual's access was delayed, the first mover usually took all the food. In comparison, when German 4-year-olds were tested with a similar set-up, they mostly retrieved the things they had worked for and respected their partner's claims (Kanngiesser et al., Reference Kanngiesser, Rossano, Frickel, Tomm and Tomasello2020). To date, there seems to be no convincing evidence for non-human primates that would fit Boyer's ownership psychology (but clearly more studies are needed).

One can assume, nevertheless, that both systems that Boyer requires for his model are present in non-human primates. Numerous studies have shown that non-human primates are able to cooperate under some conditions (Duguid & Melis, Reference Duguid and Melis2020). Yet, unlike in humans (if Boyer's model is correct), the systems for competitive resource acquisition and for cooperative interactions may not be linked in non-human primates and operate independently. It is possible that Boyer would agree with this assessment. But even if he does not agree, his model may still inspire further comparative research on possession and ownership psychology in humans and non-human primates.