Carl Hart is a Columbia University professor of psychology and neuroscience. He chairs the psych department and has a fondness for heroin – not only as a subject of scholarly pursuit but also as a substance for personal use. At 54, the married father of three has snorted small amounts of heroin for as many as 10 days in a row and enjoyed it mightily – even if, as he recalls in his new book “Drug Use for Grown-ups: Chasing Liberty in the Land of Fear” (Penguin Press), he’s experienced mild withdrawal symptoms “12 to 16 hours after the last...