Moreover, the study found that BPD was estimated to be the third most-heritable personality disorder out of the 10 personality disorders reviewed. Twin, sibling, and other family studies indicate partial heritability for impulsive aggression, but studies of serotonin-related genes have suggested only modest contributions to behavior.Families with twins in the Netherlands were participants of an ongoing study by Trull and colleagues, in which 711 pairs of siblings and 561 parents were examined to identify the location of genetic traits that influenced the development of BPD. Research collaborators found that genetic material on chromosome 9 was linked to BPD features. The researchers concluded that "genetic factors play a major role in individual differences of borderline personality disorder features". These same researchers had earlier concluded in a previous study that 42% of variation in BPD features was attributable to genetic influences and 58% was attributable to environmental influences. Genes under investigation as of 2012 include the 7-repeat polymorphism of the dopamine D4 receptor (DRD4) on chromosome 11, which has been linked to disorganized attachment, whilst the combined effect of the 7-repeat polymorphism and the 10/10 dopamine transporter (DAT) genotype has been linked to abnormalities in inhibitory control, both noted features of BPD. There is a possible connection to chromosome 5. Brain abnormalities
A number of neuroimaging studies in BPD have reported findings of reductions in regions of the brain involved in the regulation of stress responses and emotion, affecting the hippocampus, the orbitofrontal cortex, and the amygdala, amongst other areas.
While movies and visual media often sensationalize people with BPD by portraying them as violent, the majority of researchers agree that people with BPD are unlikely to physically harm others. Although people with BPD often struggle with experiences of intense anger, a defining characteristic of BPD is that they direct it inward toward themselves. One of the key differences between BPD and antisocial personality disorder (ASPD) is that people with BPD tend to internalize anger by hurting themselves, while people with ASPD tend to externalize it by hurting others.One 2020 study found that BPD is individually associated with psychological, physical and sexual forms of intimate partner violence (IPV), especially amongst men. In terms of the AMPD trait facets, hostility (negative affectivity), suspiciousness (negative affectivity) and risk taking (disinhibition) were most strongly associated with IPV perpetration for the total sample.In addition, adults with BPD have often experienced abuse in childhood, so many people with BPD adopt a "no-tolerance" policy toward expressions of anger of any kind. Their extreme aversion to violence can cause many people with BPD to overcompensate and experience difficulties being assertive and expressing their needs. This is one way in which people with BPD choose to harm themselves over potentially causing harm to others. Another way in which people with BPD avoid expressing their anger through violence is by causing physical damage to themselves, such as engaging in non-suicidal self-injury.