Trust and verify

About those cheap mental scanners, there's this paper from last week by Damon Tomlin and colleagues, which relied on brain scans to determine the cortical correlates of exchange:

We carried out an iterated, two-person economic exchange and made simultaneous hemodynamic measurements from each player's brain. These joint measurements revealed agent-specific responses in the social domain ("me" and "not me") arranged in a systematic spatial pattern along the cingulate cortex. This systematic response pattern did not depend on metrical aspects of the exchange, and it disappeared completely in the absence of a responding partner.

So, supposing that simultaneous hemodynamic measurements became cheap and easy, you could figure out if someone was cheating you.

In any event, the first two paragraphs of this paper are a very useful mini-review on social exchange and cognition:

Social exchange occurs in species ranging from insects to humans (1-3). In primates, reciprocal interactions with nonkin occur repeatedly, thus necessitating the capacity to assign social credit or blame for shared outcomes and to act appropriately according to these assignments (4-6). In humans, reciprocity is a central feature of the collection of psychological mechanisms necessary to support social exchange (3); yet, the underlying neural representations of these mechanisms remain murky. In almost all social exchanges, one must detect and accurately track which social agent (who) gets credit for an outcome. Should credit for an outcome be assigned to one's own actions or those of one's partner? Perhaps such assignments are more a matter of degree--assigning the degree-of-credit to some shared outcome. Understanding such agent-specific mechanisms is important, because the assignment of social agency (7-13) appears to break down in a range of mental illnesses (14-16).
Social agency computations are also a prerequisite for generating models of others's mental states. This latter capacity, called theory-of-mind, is highly developed in humans and has been shown to activate a consistent set of brain regions in neuroimaging experiments (17-20). Recent work has complemented these theory-of-mind experiments by using interactive economic games as ecologically realistic models for human exchange (21-31). These experiments have elicited not only brain responses in previously described theory-of-mind networks (27-29), but also have elicited formerly unreported activations along the cingulate cortex that correlate with the revelation of a social partner's decision (29). Although evoked during an economic exchange with another human, these cingulate activations did not modulate as a function of the fairness of the exchange, nor did they occur in exchanges with computer partners (28).

References:

Tomlin D et al. 2006. Agent-specific responses in the cingulate cortex during economic exchanges. Science 312:1047-1050. DOI link