Borderline personality disorder (BPD) does not have a single known cause; most experts think it develops from a mix of biological vulnerability and adverse life experiences. Having those risk factors does not mean someone will definitely develop BPD, and people can have BPD without obvious trauma.
Biological factors
Research suggests that genes play a role, because BPD is more common in people who have close relatives with the condition or other related mental health disorders. Studies also show differences in brain systems involved in emotion regulation, impulse control, and aggression, including changes in neurotransmitters such as serotonin and in areas like the amygdala and prefrontal cortex.
Childhood experiences and trauma
Many people diagnosed with BPD report difficult early environments, such as emotional, physical, or sexual abuse, neglect, chronic fear or instability at home, or growing up with a caregiver who had serious mental health or substance use problems. These experiences can affect attachment, sense of safety, and how emotions are expressed and responded to, which may contribute to later problems with identity, trust, and emotional regulation.
Psychosocial and environmental factors
Beyond severe trauma, BPD has been linked to more subtle but chronic invalidation, such as having feelings dismissed, ridiculed, or ignored, or growing up in families with high conflict and poor communication. The leading “biosocial” theory describes BPD as emerging when an emotionally sensitive person repeatedly encounters environments that do not understand or support their emotional needs.
Interaction of factors
Current thinking is that BPD typically develops from an interaction: inherited or biological emotional sensitivity increases vulnerability, and stressful or traumatic experiences then activate or shape that vulnerability over time. Because these factors interact in complex ways, two people with very similar histories may differ—one may develop BPD while another does not, and many people with BPD can improve significantly with appropriate treatment and support.
