“Less than schizophrenia but ahead of lots of other psychiatric disorders,” says Palmer about the role genes play in the genesis of borderline personality disorder. “The condition is now believed to be 55 percent heritable.” Increasingly, the origins of the condition are seen as a classic interplay of nature and nurture.
The parental role is complex, says Gunderson. Children who develop BPD inherit a temperament—one that makes them highly reactive, emotional, and so hypersensitive to perceived anger or rejection they might cry inconsolably if scolded—that can tax even a good caretaker. “The hostile, conflicted relationships that evolve are not, as traditionally thought, a result of poor parents, but of parents whose parenting is shaped by a difficult child. It might take an extraordinarily calm parent to keep a genetically loaded infant from developing the disorder.”
Researchers have identified unusually heightened activity in the amygdala, a brain structure that forms part of the limbic system, which governs memory and the sense of smell as well as emotional reactivity. They believe the reactivity gives rise to a hair-trigger temper. In addition, many borderline patients have a specific short variant of the serotonin transporter, or 5-HTT, gene. It affects how much neurotransmitter is available to nerve cells, and the short allele has been linked to anxious, aggressive, and impulsive behavior.
But abusive parenting and other traumatic childhood experiences still seem to figure into the disorder. A large number of sufferers do, in fact, have incidents of physical or emotional abuse in their past, although in some cases they may be the result of a difficult-to-manage temperament, not its cause.
Psychiatrist Otto Kernberg of New York’s Weill Cornell Medical College, one of the first researchers to describe the borderline personality, has long seen an overly rigid approach to life as a consistent feature. In his view, it evolves out of direct experience of physical or emotional abuse or witnessing others being abused, though he acknowledges the contribution of such biological defects as an overactive amygdala. He finds that borderline patients have a tendency to separate experiences into “positive” and “negative” buckets in their mind—a maneuver they engage in, he says, to prevent positive experiences from being contaminated by negative ones. A person may cling to sunny memories of his mother buying him an ice cream cone, for example, even though she abandoned him later on.
As they mature, borderlines continue to idealize some things and demonize others to make sense of a world that seems frightening. “There’s a lack of capacity for a realistic assessment,” says Kernberg. A friend who merited endless love on Monday could be persona non grata by Tuesday because she turned down an invitation to coffee.
Perhaps as part of an attempt to cope with abuse, borderline patients may have a distorted perception of time, says San Diego’s David Reiss. They see it more as an accumulation of distinct events than a continuous linear progression. It leads to difficulty in perceiving the chronological sequence of events. The misperception of time may compound the problems borderlines face in fulfilling life responsibilities.