Successful human social life requires imagining what others believe or think to understand and predict behavior. This ability, often referred to as theory of mind (ToM), reliably engages a specialized network of temporal and prefrontal brain regions in older children and adults, including selective recruitment of the temporal-parietal junction (TPJ). To date, how and when this specialized brain organization for ToM arises is unknown due to limitations in functional neuroimaging at younger ages. Here, we used the emerging technique of functional near-infrared spectroscopy to measure the functional brain response across parietal, temporal, and prefrontal regions in 7-month-old male and female infants as they viewed different video scenarios of a person searching for a hidden object. Over different conditions, we manipulated whether the person held an accurate (true) or inaccurate (false) belief about the location of the hidden object in the videos. In two separate experiments, we observed that activity from the TPJ, but not other temporal and prefrontal regions, spontaneously tracked with the beliefs of the other person, responding more during scenarios when the other person's belief regarding the location of the object was false compared with scenarios when her belief was true. These results mirror those obtained with adults to show that the TPJ already shows some functional organization relevant to high-level social cognition by around 7 months of age. Furthermore, these results suggest that infants may draw on similar core mechanisms to implicitly track beliefs, as adults do when reasoning explicitly about them. Humans selectively engage a network of brain regions, including the temporal-parietal junction (TPJ), to track what others think, an ability referred to as theory of mind. How and when this specialized brain organization for high-level social cognition arises is unknown. Using the emerging technique of near-infrared spectroscopy with 7-month-old infants, we observed that activity of the TPJ, but not other temporal and frontal regions, distinguished between scenarios when another person's belief about the location of the object was false compared with scenarios when the belief was true. These results suggest that a basic neural architecture to understand and predict the actions of others based on their beliefs may be present from the first year of life.

Adults and older children are more likely to punish a wrongdoer for a moral transgression when the victim belongs to their group. Building on these results, in violation-of-expectation experiments (n= 198), we examined whether 2.5-year-old toddlers (Exps. 1 and 2) and 1-year-old infants (Exps. 3 and 4) would selectively expect an individual in a minimal group to engage in third-party punishment (TPP) for harm to an ingroup victim. We focused on an indirect form of TPP, the withholding of help. To start, children saw a wrongdoer steal a toy from a victim while a bystander watched. Next, the wrongdoer needed assistance with a task, and the bystander either helped or hindered her. The group memberships of the wrongdoer and the victim were varied relative to that of the bystander and were marked with either novel labels (Exps. 1 and 2) or novel outfits (Exps. 3 and 4). When the victim belonged to the same group as the bystander, children expected TPP: At both ages, they detected a violation when the bystander chose to help the wrongdoer. Across experiments, this effect held whether the wrongdoer belonged to the same group as the bystander and the victim or to a different group; it was eliminated when the victim belonged to a different group than the bystander, when groups were not marked, and when either no theft occurred or the wrongdoer was unaware of the theft. Toddlers and infants thus expect individuals to refrain from helping an ingroup victim’s aggressor, providing further evidence for an early-emerging expectation of ingroup support.

Successful human social life requires imagining what others believe or think to understand and predict behavior. This ability, often referred to as theory of mind, reliably engages a specialized network of temporal and prefrontal brain regions in older children and adults, including selective recruitment of temporal-parietal junction (TPJ). To date, how and when this specialized brain organization for ToM arises is unknown due to limitations in functional neuroimaging at younger ages. Here we employed the emerging technique of functional near-infrared spectroscopy (fNIRS) to measure the functional brain response across the parietal, temporal, and prefrontal regions in 7-month old male and female infants as they viewed different video scenarios of a person searching for a hidden object. Over different conditions, we manipulated whether the person held an accurate (true) or inaccurate (false) belief about the location of the hidden object in the videos. In two separate experiments, we observed that the TPJ, but not other temporal and prefrontal regions, spontaneously tracked with the beliefs of the other person, responding more during scenarios when the other person’s belief regarding the location of the object was false compared to scenarios when her belief was true. These results mirror those obtained with adults to show that the TPJ already shows some functional organization relevant to high-level social cognition by around 7-months. Furthermore, these results suggest that infants may draw on similar core mechanisms to implicitly track beliefs as adults do when explicitly reasoning about them.

By 2 y of age, children possess expectations about several different moral principles. Building on these results, we asked whether children who observed a wrongdoer violate a principle would draw negative inferences from this violation about how the wrongdoer was likely to behave in other contexts. In four experiments, 25-mo-old toddlers (n = 152) first saw a wrongdoer harm a protagonist. When toddlers judged the wrongdoer’s behavior to violate the principle of ingroup support or harm avoidance, they did not find it unexpected if the wrongdoer next violated the principle of fairness by dividing resources unfairly between two other protagonists (Exps. 2 and 3), but they did find it unexpected if the wrongdoer next acted generously by giving another protagonist most of a resource to be shared between them (Exp. 4). When toddlers did not construe the wrongdoer’s harmful behavior as a moral violation, these responses reversed: They found it unexpected if the wrongdoer next acted unfairly (Exp. 1) but not if the wrongdoer next acted generously (Exp. 4). Detecting a moral violation thus lowered toddlers’ assessment of the wrongdoer’s moral character and brought down their expectations concerning the likelihood that the wrongdoer would perform: 1) obligatory actions required by other principles and 2) supererogatory or virtuous actions not required by the principles. Together, these findings expand our understanding of how young children evaluate others’ moral characters, and they reveal how these evaluations, in turn, enable children to form sophisticated expectations about others’ behavior in new contexts.

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