Abstract
A persistent methodological problem in primate social cognition research has been how to determine experimentally whether primates represent the internal goals of other agents or just the external goals of their actions. This is an instance of Daniel Povinelli’s more general challenge that no experimental protocol currently used in the field is capable of distinguishing genuine mindreading animals from their complementary behavior-reading counterparts. We argue that current methods used to test for internal-goal attribution in primates do not solve Povinelli’s problem. To overcome the problem, a new type of experimental approach is needed, one which is supported by an alternative theoretical account of animal mindreading, called the appearance-reality mindreading (ARM) theory. We provide an outline of the ARM theory and show how it can be used to design a novel way to test for internal-goal attribution in chimpanzees. Unlike protocols currently in use, the experimental design presented here has the power, in principle and in practice, to distinguish genuine mindreading chimpanzees from those who predict others’ behavior solely on the basis of behavioral/environmental cues. Our solution to Povinelli’s problem has important consequences for a similar debate in developmental psychology over when preverbal infants should be credited with the ability to attribute internal goals. If what we argue for here in the case of nonhuman primates is sound, then the clearest tests for internal-goal attribution in infants will be those that test for attributions of discrepant or ‘false’ perceptions.
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Notes
For convenience, nonhuman animals and nonhuman primates will henceforth be referred to as animals and primates.
Note that grasping an object is not a mental state of the agent, in contrast to the agent’s intending or having the goal to grasp the object, which are mental states.
Vonk and Povinelli overstate the problem here a bit. It should not be assumed that a behavior-reading animal’s prediction of an agent’s behavior must involve only a single inferential step—a step from the observed behavioral/environmental cue to the predicted behavior. There is no reason why the behavior-reading animal could not engage in several inferential steps. The important point is that whatever further steps the animal may use, they will not involve the attribution of mental states.
In the above quotation, Vonk and Povinelli represent assumption (1) here with the notation ‘Sb+ms’.
As should be clear from the above discussion, not all instances of behavior-reading fit into the category of complementary behavior-reading. In many instances, animals learn (or innately know how) to anticipate other agents’ behavior (e.g., fleeing, feeding, mating) on the basis of certain behavioral/environmental cues (e.g., forward-facing torso, sound of a bell, smell of sex pheromones in conspecifics, etc.) that are not in any plausible way indicative of kinds of mental states in the other agent. Lurz (2009, 2011b) calls such non-mindreading methods of behavior prediction minimal behavior-reading (for evidence of this in chimpanzees see Povinelli and Eddy 1996; Vonk and Subiaul 2009). The categories of mindreading and complementary behavior-reading are, therefore, not exhaustive; an animal can be a minimal behavior-reader and fail to be in either of these categories. Thus, it bears reiterating that it is the realistic possibility that chimpanzees (and perhaps other primates) are complementary behavior-readers—not minimal behavior-readers—that creates Povinelli’s problem for chimpanzee (primate) theory-of-mind research.
See Povinelli et al. (1998) for a similar study that produced negative results.
All the chimpanzees tested were captive born with a mean age of 15.4 years. Thus, by the time they were tested, they would have had plenty of opportunity to learn such response strategies as a result of their interactions with human caretakers who fed them.
A similar complementary behavior-reading explanation can be given for Phillip et al.’s (2009) study with capuchin monkeys. In one of their experiments, however, the researchers used a spoon, rather than a human hand, to deliver food to the animal subject during motivational trials. In contrast to their differential responses in human-hand experiments, monkeys in the spoon experiment did not discriminate between a spoon that was ‘unable yet willing’ and one that was ‘unwilling yet able’ to deliver food. In each case, the monkeys behaved impatiently and quickly left the testing room. The researchers argue that the monkeys’ differential responses to the spoon and the human hand provided evidence of goal-attribution when it comes to humans but not to inanimate objects. However, such results are in no way inconsistent with what one would expect from a complementary behavior-reading account of the monkeys’ behavior. For the complementary behavior-reading hypothesis does not say that monkeys learn to follow behavioral rules that apply to any moving object (even spoons). Rather, the hypothesis holds, just as does the mindreading hypothesis that the researchers endorse, that monkeys follow rules that they learned from (and take to apply specifically to) human or other animal agents. Thus, the complementary behavior-reading hypothesis is no more expected to predict a differential response pattern from monkeys in the different spoon trials than is the mindreading hypothesis endorsed by Phillip and colleagues.
Similar results were found with two other apparatuses that the demonstrator also illuminated using an unusual body part, although no significant differences were found with apparatuses that produced sounds (Buttelmann et al. 2007).
See Rochat et al. (2008) for a similar violation-of-expectancy, looking-time study with macaques.
The researchers did observe that the monkey’s grasp-and-eat neurons also fired when it observed the experimenter grasping a grape in the context of a container, but they did not fire as strongly as its grasp-and-place neurons (p. 664).
Those familiar with research on perceptual-state attribution in primates might be inclined to object here, pointing out that chimpanzees have already demonstrated that they can attribute such internal goals to other agents. A number of researchers, for example, interpret studies by Hare et al. (2000, 2001) as providing some of the strongest evidence that chimpanzees can predict dominant conspecifics’ feeding behaviors on the basis of whether they think the dominant sees the food in the environment or saw where the food was last hidden. However, Heyes (1994, 1998), Lurz (2009), and Povinelli and Vonk (2006) have persuasively argued that these studies, as well as other perceptual-state attribution studies in primates and other animals, are unable to determine whether chimpanzees predict other agents’ behavior on the basis of what they think the agent sees or cannot see, or simply on the basis of what they think the agent has or does not have an unobstructed line of gaze to (either currently or in the recent past). Having a line of gaze to a distal object is not a psychological state, and it is not the psychological state of seeing the object (e.g., blind people can and often do have unobstructed lines of gaze to objects in their environment; they just don’t see the objects). However, line of gaze is one of the principal observable cues on which an animal might reasonably infer that another subject sees an object. The main objective of this paper is to show how researchers can overcome this methodological problem by running a type of test in which an animal can predict an agent’s external goal by attributing a perceptual state (internal goal) to the agent but not by representing facts about what the agent has (or lacks) a line of gaze to.
This is not to say that animals do not have spatially internal, causally efficacious, representational states by virtue of which they think, reason, perceive, and attribute mental states to others. What we are claiming is that, in their ability to attribute mental states (even discrepant or ‘false’ mental states), animals do not need to be understood to represent mental states in such ways. Representing mental states in such terms may be the way that we (philosophers, scientific researchers, and perhaps most people) represent them, but it should not be assumed that it is the way that animals represent them.
An interesting feature of the triadic-relation model of perceptual-state attributions is that some animals—most notably apes (see Tomasello and Call 1997)—have been shown to understand other types of triadic relations, both in the social domain (e.g., understanding the relation of their being between individuals x and y in dominance rank) and in the domain of tool use (e.g., understanding the relation between a demonstrator, a tool, and the object the tool is used to manipulate).
On representational theories of perception, the truth-evaluable contents of perceptual states of the form ‘x looks F to S’ are often identified with propositions (e.g., the proposition that x is F; see Armstrong 1968; Thau 2002) or, on some accounts, with non-propositional, truth-evaluable entities such as positioned scenarios (e.g., the set of different ways of filling out the space before S’s body that are consistent with the fact that x looks F to S; see Peacocke 1992).
A follow-up test was also run to rule out the possibility that chimpanzees were solving the task by simply keeping track of which container they saw the large grape being placed into. Five chimpanzees (more than a third of the original sample) also passed this test.
This is important to note since in the test trials, it will be critical that the competitor cannot see the grapes directly but only by looking at them through the sides of the glass containers.
The results to examine will be those from the first few competitor-first size-distorting trials. Chimpanzees’ responses in these trials will give the clearest indication of whether they show the predicted anticipation of the competitor’s reaching. Positive results from later trials may reveal only that chimpanzees have learned to anticipate such selection behavior from the competitor through their experience in previous trials.
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We wish to thank the anonymous reviewers for their insightful and helpful comments on an earlier draft of this paper, and to thank Kristy Lee for drawing Fig. 1.
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Lurz, R.W., Krachun, C. How Could We Know Whether Nonhuman Primates Understand Others’ Internal Goals and Intentions? Solving Povinelli’s Problem. Rev.Phil.Psych. 2, 449–481 (2011). https://doi.org/10.1007/s13164-011-0068-x
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DOI: https://doi.org/10.1007/s13164-011-0068-x