“The statistics on sanity are that one out of every four Americans is suffering from some form of mental illness. Think of your three best friends. If they're okay, then it's you.”
Many sources attribute this quote to Brown without giving a specific reference to her writings. The earliest located is the following variation from p. 47 of Musgrave Landing: Musings on the Writing Life by Susan Musgrave (1994), which Musgrave quotes as "Rita Mae Brown's warning": "If you become the kind of writer who calls forth heated emotional states, be careful. There are a lot of unbalanced people out there. The statistics on insanity are that one out of every four people is suffering from some form of mental illness. Think of your three best friends. If they're okay, then it's got to be you."
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Rita Mae Brown 38
Novelist, poet, screenwriter, activist 1944Related quotes
Source: If You Want to Write: A Book about Art, Independence and Spirit

Quoted from his first book https://en.wikipedia.org/wiki/Success_and_Failure_Based_on_Reason_and_Reality, "Success and Failure Based on Reason and Reality" https://www.amazon.co.uk/SUCCESS-FAILURE-BASED-REASON-REALITY/dp/9970983903/ on Amazon, P.58 (July 2018)
Source: From the Notebooks of Dr. Brain (2007), Chapter 2 “Facing the Ultimate Archenemy” (p. 57)

“My best friends are my memories
They're the only ones that save me from misery”
Source: Song Tear me down