People with borderline personality disorder have a harder time separating their own actions and thoughts from others'.

People with borderline personality disorder, or BPD, often struggle in relationships. Researchers think this happens because they have trouble with self-other distinctionself-other distinctionThe ability to keep your own thoughts, feelings, and actions separate from someone else's., which is the brain's ability to separate your own thoughts and actions from someone else's. To test this, scientists compared people with BPD to healthy adults on specific computer tasks. One test checked if people could stop themselves from automatically copying movements they saw. Another checked if they could step outside their own visual viewpoint to see what someone else sees. Overall, participants with BPD were slower and less accurate. They showed lower physical control, meaning they were more easily distracted into mimicking others. They also showed a stronger self-centered bias when trying to see another person's view, though this effect depended on other factors. Finally, in BPD, having more severe symptoms linked to this viewpoint bias specifically when people had also experienced more childhood maltreatment.

Why it matters

For the millions of people with borderline personality disorder, everyday interactions can feel confusing—they may unconsciously mimic others or assume others see things exactly as they do. This study shows these difficulties are not a single global problem but appear in specific ways, and that early trauma makes the cognitive version worse. That matters because it points toward more targeted treatments, like training to recognize and resist automatic imitation or to consciously adopt another person's viewpoint.

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