We know that BPD does tend to run in families, but for a long time it was not known whether this was because of environmental influences or because of genetics. There is now some evidence that genetics do play a significant role, but environment is also important.
Studies have shown that a variation in a gene which controls the way the brain uses serotonin (a natural chemical in the brain) may be related to BPD, particularly if the individual with this gene variation experiences stressful childhood events (e.g., trauma or separation from supportive caregivers). For example, one study found that monkeys with the serotonin gene variation developed symptoms that looked similar to BPD only when they were taken from their mothers and raised in less nurturing environments. Monkeys with the gene variation who were raised by nurturing mothers were much less likely to develop BPD-like symptoms. Learn more about the influence of genes and environment on the development of BPD.
Lis E, Greenfield B, Henry M, Guile JM, Dougherty G. "Neuroimaging and Genetics in Borderline Personality Disorder: A Review." Journal of Psychiatry and Neuroscience, 32:162-173, 2007.