Psychotic illness is a disorder of social interaction unique to humans. However, up to now research has failed to pin down the exact determinants of the complex and interactive processes associated with the development of trust and reciprocity in psychosis. Utilizing a novel multi-round version of an interactive trust game experiment, we show that patients with psychosis and healthy relatives with a heightened risk for the illness exhibit lower baseline levels of trust compared with healthy controls. This effect partly overlapped with a reduced general intelligence. Furthermore, patients were unable to modify their trusting behaviour neither in response to information about the general trustworthiness of their interaction partner, nor in response to their partners' specific direct behavioural feedback. Relatives, in contrast, modified their trusting behaviour towards similar levels as healthy subjects in response to both. The results show that behavioural flexibility in response to socially relevant information is a critical determinant of success in the instantiation and maintenance of social relationships. A lack thereof may drive social dysfunction and the progression from subclinical symptoms to a full-blown psychosis. This offers a testable mechanistic hypothesis for progression from prodrome to psychotic illness, and may provide a therapeutic avenue to grapple the psychotic symptoms of social dysfunction.

Psychosis is characterized by an elementary lack of trust in others. Trust is an inherently rewarding aspect of successful social interactions and can be examined using neuroeconomic paradigms. This study was aimed at investigating the underlying neural basis of diminished trust in psychosis. Functional magnetic resonance imaging data were acquired from 20 patients with psychosis and 20 healthy control subjects during two multiple-round trust games; one with a cooperative and the other with a deceptive counterpart. An a priori region of interest analysis of the right caudate nucleus, right temporo-parietal junction and medial prefrontal cortex was performed focusing on the repayment phase of the games. For regions with group differences, correlations were calculated between the haemodynamic signal change, behavioural outcomes and patients' symptoms. Patients demonstrated reduced levels of baseline trust, indicated by smaller initial investments. For the caudate nucleus, there was a significant game × group interaction, with controls showing stronger activation for the cooperative game than patients, and no differences for the deceptive game. The temporo-parietal junction was significantly more activated in control subjects than in patients during cooperative and deceptive repayments. There were no significant group differences for the medial prefrontal cortex. Patients' reduced activation within the caudate nucleus correlated negatively with paranoia scores. The temporo-parietal junction signal was positively correlated with positive symptom scores during deceptive repayments. Reduced sensitivity to social reward may explain the basic loss of trust in psychosis, mediated by aberrant activation of the caudate nucleus and the temporo-parietal junction.

Changes in social behaviour from childhood to adulthood have been suggested to be driven by an increased sensitivity to others' perspectives. Yet, the link between perspective‐taking and social processes, such as trust and reciprocity, has rarely been investigated during adolescence. Using two trust games with a cooperative and an unfair counterpart and an online perspective‐taking task with 50 adolescents, we show that those with a higher perspective‐taking tendency demonstrate greater trust towards others and higher levels of trust during cooperative interactions. Both low and high perspective‐takers adapted their levels of trust in response to unfair behaviour. However, high perspective‐takers reduced their trust more drastically and showed more malevolent and less benevolent tit‐for‐tat when they were treated unfairly by their counterpart. The findings suggest that a higher perspective‐taking tendency in adolescence is associated with specific mechanisms of trust and reciprocity, as opposed to undifferentiated increases in positive social behaviour towards others.

The tendency to trust and to cooperate increases from adolescence to adulthood. This social development has been associated with improved mentalizing and age-related changes in brain function. Thus far, there is limited imaging data investigating these associations. We used two trust games with a trustworthy and an unfair partner to explore the brain mechanisms underlying trust and cooperation in subjects ranging from adolescence to mid-adulthood. Increasing age was associated with higher trust at the onset of social interactions, increased levels of trust during interactions with a trustworthy partner and a stronger decline in trust during interactions with an unfair partner. Our findings demonstrate a behavioural shift towards higher trust and an age-related increase in the sensitivity to others' negative social signals. Increased brain activation in mentalizing regions, i.e. temporo-parietal junction, posterior cingulate and precuneus, supported the behavioural change. Additionally, age was associated with reduced activation in the reward-related orbitofrontal cortex and caudate nucleus during interactions with a trustworthy partner, possibly reflecting stronger expectations of trustworthiness. During unfair interactions, age-related increases in anterior cingulate activation, an area implicated in conflict monitoring, may mirror the necessity to inhibit pro-social tendencies in the face of the partner's actual levels of cooperation.