Brooks King-Casas and colleagues recruited dozens of people with borderline personality disorder (BPD) to play the role of trustee in an economic game. People with this disorder tend to have unstable personal relationships and difficulty regulating their emotions. ...
Over several rounds, the game involved the investor choosing how much money to pass to the trustee. The investment was automatically tripled and then the trustee had to decide how much money to pass back to the investor. For maximum returns, both parties need to cooperate. If the trustee is unfair in the returns he gives back, the investor will likely reduce his investments on future rounds, meaning less profit for everyone.
The researchers found that cooperation broke down when a person with BPD played the role of trustee. They failed to recognise smaller investments as a sign that the investor was losing trust. Healthy trustees, by contrast, responded to a distrustful investor by increasing the returns they gave, thereby coaxing back the investor's trust and provoking a return to larger investments.
Brain scans taken while the participants played the role of trustee showed that healthy participants, but not participants with BPD, showed greater activity in the anterior insula as investments reduced in size (this is a brain region known to be involved in fairness, as well as sensing the body's internal states). Perhaps because of their low expectations for how others will treat them, the participants with BPD didn't appear to recognise a low investment [return] as unfair.
--Research Digest blog on why BPD people are so difficult to interact with