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Borderline Personality Disorder Sheds Light on How We Decide Who to Trust

How do we decide who to trust? Is it a reasoned decision or an instinctual response to potential threats?

That question was part of a new study on borderline personality disorder by Professor Eric Fertuck (The City College of New York, The Graduate Center) and colleagues at Columbia University-New York State Psychiatric Institute.

Individuals with borderline personality disorder (BPD) have difficulty establishing stable, trusting relationships, and they fear abandonment from significant others. They’re prone to feelings of betrayal and misplaced trust in others. Their heightened sensitivity to “social threats” has been attributed to hyperactivity in the brain’s amygdala region, which detects and responds to threatening situations. Fertuck’s new study sought to test whether amygdala hyperactivity could also explain why those with BPD are more likely than the rest of us to judge others as untrustworthy.

The study appeared in NeuroImage: Clinical. It’s part of ongoing research by Fertuck and colleagues on various aspects of BPD. The study was supported by the National Institute of Mental Health.

The study presented facial images to 16 women with BPD diagnoses and 17 healthy control subjects with no psychiatric diagnoses. The subjects judged the faces on a scale from 1 (neutral or trustworthy) to 5 (fearful or untrustworthy). Functional MRI scans determined which part of their brains were more active during the task.

The study found that BPD subjects were indeed more likely than control subjects to judge faces as untrustworthy, a finding that is consistent with previous research. But the study surprised researchers by indicating that the amygdala was not involved in the trustworthiness appraisal. That was true for both BPD and non-BPD participants.

Instead, researchers found that judging trustworthiness activated the prefrontal cortex, the region of the brain that makes decisions and distinguishes good from bad. At the same time, the BPD subjects showed less prefrontal activity than the control group. That suggests an impaired ability to make a reasoned appraisal of trustworthiness in those with BPD.

The takeaway: Deciding whether someone is trustworthy is less of an automatic response to fear (involving the amygdala) than it is a reasoned appraisal in a social context, based on judgments of whether the person might exploit or betray us. The BPD group showed less prefrontal activity than others as they decided who to trust, and that might explain why their judgment was faulty.

The findings also have implications for BPD treatment. Instead of just focusing on moderating “amygdala-driven emotional hyper-reactivity,” clinicians might work on “improving accurate probabilistic reasoning around trustworthiness appraisals.”