Psychiatrists and mental health experts are asking that a rule be made that any presidential candidate over the age of 75 have a “comprehensive neuropsychological evaluation” to ensure that he or she is fit to serve in office. The working group, known as “Duty to Warn,” collaborated in 2017 on a book that walked through the mental and cognitive health of then-President Donald Trump and the dangers he posed to the United States. There is a rule against diagnosing public figures. In the case of Trump, the “Duty to Warn” group was so-named because it is part of the A.P.A.’s ethics rules if someon…