How do we know how prevalent BPD is?
First, we should note that in the early 1990s BPD was not recognized as a common disorder. Now it is.
A number of studies have been performed in the USA to determine prevalence. Some found that 1.8% of Americans meet the criteria, while others suggested a range of 2-4% or even almost 6%. Even the lowest estimate finds that BPD is about as common as Bi-Polar I and more common than Schizophrenia, both of which are far better known.
Summing up the conservative end, Zanarini stated in 2016: 1.8% of Americans meet criteria. (Zanarini McLean Webinar–BPD Patient and Family Education Initiative, “The Long-Term Course of Borderline Personality Disorder,” Originally aired Wednesday, February 3, 2016, accessed at https://www.mcleanhospital.org/borderline-personality-disorder-patient-and-family-education-initiative)
In an earlier 2003 study Zanarini had stated that clinical experience suggests that BPD is often underdiagnosed in many treatment settings, so 1.8% may be low. (Zanarini et al. “A SCREENING MEASURE FOR BPD: THE MCLEAN SCREENING INSTRUMENT FOR BORDERLINE PERSONALITY DISORDER (MSI-BPD)” Journal of Personality Disorders, 17, 568-573, 2003.)
Gunderson, championing the middle road, states that BPD is the most common of the personality disorders, with a prevalence rate of approximately 2–4% in the general population, accounting for 20% of people receiving inpatient mental health care and 15% of those receiving outpatient care. (Gunderson, J. . A BPD brief: An introduction to borderline personality disorder: Diagnosis, origins, course, and treatment. Retrieved from www.borderlinepersonalitydisorder.com/professionals/a-bpd-brief/).
Grant et al provide the highest figures:
“The results of a large, well-conducted study funded by the National Institute of Mental Health suggests that the lifetime prevalence rate of borderline disorder is 5.9% of the general population and appears to occur equally in men and women.” (Grant et al. “Prevalence, Correlates, Disability, and Comorbidity of DSM-IV Borderline Personality Disorder: Results from the Wave 2 National Epidemiologic Survey on Alcohol and Related Conditions” J Clin Psychiatry. 2008 Apr; 69: 533–545. Summarized in Robert O. Friedel M.D., accessed at http://www.bpddemystified.com/overview/ , January 17, 2019.)