National Institute of Mental Health free of mental illness 26% of the adult population, each year 26% of the adult population, each year I would likely have guessed something like 10% Mental Illness Statistics leading cause one in four adults 26% of the adult population, each year 26% of the adult population, each year 26% of the adult population, each year 26% of the adult population, each year US Statistics For Mental Disorder Cases 26% of the adult population, each year 26% of the adult population, each year 26% of the adult population, each year