The prevalence of Cluster B personality disorders may be influenced by various factors, such as changes in diagnostic criteria, societal attitudes towards mental health, access to mental health services, and research advancements in understanding and diagnosing these conditions. Changes in any of these factors could potentially impact the reported prevalence rates.
To get an accurate and up-to-date prediction on whether the population of Cluster B disorder will increase or decrease in the next 20 years, it would be best to refer to recent research studies and consult mental health professionals and experts in the field. They would have access to the most current data and insights to make informed assessments about potential trends in the prevalence of these disorders.