One of the many interesting applications of Boyer's theory concerns the reprehensible practice of slavery. He notes that a view of ownership intuitions as based on accepted social norms or from a mental theory of ownership would necessitate that “the abolitionist movement that started in 18th century England required a drastic change in conceptions of ownership.” Boyer argues that, instead, those in favor of retaining the practice of slavery focused on “depicting slaves as essentially different from full human beings (Smithers, Reference Smithers2012),” and that “As many historians have noted (Carey, Reference Carey2005), the emergence of abolitionism did not result from the adoption of a different mental theory or social norm about the domain of ownership, but from a widening of the ‘moral circle’ (Pinker, Reference Pinker2011; Singer, Reference Singer1981)”.

One concern, though, is that Boyer might be too quick to conflate the categories of “falling within the moral circle” and “not being property.” Consider studies that ask people to morally evaluate others and designate them as falling into different points within the moral circle. These studies find that we ascribe most moral standing to our family and friends, followed by human in-groups and outgroups, then animals, then plants, and, finally, monstrous people like murderers (see, e.g., Crimston, Bain, Hornsey, & Bastian, Reference Crimston, Bain, Hornsey and Bastian2016; Neldner, Crimston, Wilks, Redshaw, & Nielsen, Reference Neldner, Crimston, Wilks, Redshaw and Nielsen2018). As such, both animals that are seen as highly sentient (dolphins, chimpanzees, and dogs) and those seen as less sentient (chickens, fish, and bees) are closer to the center of the moral circle than human murderers and child molesters. (If anything, this underestimates how we think about certain animals. One poll finds that about one in three British pet-owners would choose to keep their pet over their romantic partner.) Yet, despite all the moral weight given to these creatures, we are fully comfortable seeing them as property. Conversely, however much we despise murderers and child molesters, we do not usually see them as entities that can be owned.

This suggests that it's not simply the degree of moral standing that we give to living beings that determines whether we view them as ownable. Instead, research suggests that the attribution of specific mental capacities may be closely linked to judgments about human and animal ownership. While some have explored the role of a capacity for intelligence (Caviola, Schubert, Kahane, & Faber, Reference Caviola, Schubert, Kahane and Faber2022; Wilks, Caviola, Kahane, & Bloom, Reference Wilks, Caviola, Kahane and Bloom2021), sentience (Gray, Young, & Waytz, Reference Gray, Young and Waytz2012; Leach, Sutton, Dhont, & Douglas, Reference Leach, Sutton, Dhont and Douglas2020; Rottman, Crimston, & Syropoulos, Reference Rottman, Crimston and Syropoulos2021), or morality (Piazza, Landy, & Goodwin, Reference Piazza, Landy and Goodwin2014), research that I've done with my colleagues and students suggests that our judgments about whether an animate being can be owned depends in large part on the degree to which we believe it has the capacity for autonomy.

One series of studies investigated the role of factors such as intelligence, emotional capacity, reflection and self-awareness, and autonomy in adults’ judgments of whether adult humans, robots, aliens, and other creatures could be owned if they were purchased by another person (Starmans & Friedman, Reference Starmans and Friedman2016). Among these capacities, only autonomy had a significant impact on ownership judgments. Participants were less likely to judge that any of these entities were owned when they were described as having the capacity to make their own decisions, resist the instructions of others, and being responsible for their own actions.

Why might autonomy be special? One proposal, in line with arguments made by Locke (Reference Locke1690) and others, is that autonomous beings might be seen as possessing the property of self-ownership. If you own yourself, then you cannot be owned by another. Consistent with this, Starmans and Friedman (Reference Starmans and Friedman2016) found that the one circumstance in which a human was viewed as owned was when the transaction respected the autonomy of the person: An adult who willingly sold himself into slavery for his own reasons was judged to be owned by the purchaser.

Does this autonomy principle apply to living beings more broadly? Espinosa and Starmans (Reference Espinosa and Starmans2020) showed that even 4- to 7-year-old children use principles of control and autonomy to reason about the ownership of familiar and novel animals. At all ages tested, children were more likely to say that a typically wild animal (e.g., a bear) or a novel alien animal was owned if a homeowner had controlled its movements by putting it in a cage, or if the animal inherently had the ability to escape the backyard (e.g., fly or jump).

This autonomy principle also has interesting implications for how human children are viewed, since young children lack autonomy, and are not viewed as having the capacity to make decisions for themselves. As such, autonomy-based reasoning might lead to the intuition that children are owned by their parents. While modern adults are unlikely to explicitly endorse this claim, Starmans & Friedman (Reference Starmans and Friedmanunder review) explored the origins of these intuitions by asking 4- to 7-year-olds about their views. We found that across all ages, children judged that parents owned children, but children did not own parents, teachers did not own students, and students did not own teachers. The one exception was when children considered a “Pippi Longstocking”-like character, who was autonomous, lived next door to her parents, and made all her own decisions. Now, she was not seen as owned by her parents.

One question that arises from this analysis is why adults do not affirm that children, or physically or mentally incapacitated adults, can be owned. We explain this in terms of an overriding general principle that modern adults (at least weird ones) possess: With certain exceptions, such as when a person chooses to be owned, people cannot be owned, regardless of how autonomous they are. The ownership of people isn't merely a grotesque moral wrong, it is also a conceptual impossibility.

If this is right, how can we explain the historical change in intuitions about the ownership of people? Here, I agree with Boyer that this is not best thought of as a “drastic change in conceptions of ownership,” but I propose that instead it represents a drastic change in the conception of people – that is, in a shifting recognition that the historically enslaved individuals had the very same capacity for autonomy, and thus self-ownership, as those who would attempt to enslave them.