Kenneth Turek has managed the Neuberger Berman Mid-Cap Growth Fund since 2002, handily beating the Russell Midcap index for the past 10 years. He likes companies whose executives “underpromise and overdeliver,” and he thinks that applies to Acadia Healthcare ACHC -1.18% . Acadia provides “behavioral health services”—inpatient and outpatient psychiatric care for people with mental-health and addiction problems. After a string of acquisitions, Acadia is now the biggest publicly traded pure-play company in this arena, one of health care’s fastest-growing segments. Turek recently told Fortune why he’s adding to his position….