 MacDonald Tow's research on the survivors demonstrated that the operation often caused serious intellectual damage.When the Board of Control published their second survey in 1961 the use of psychosurgery had declined by more than half; the decline being attributed to an awareness of the risks of irreversible effects and the introduction in 1955 of neuroleptic drugs.By 1961 nearly half of mental hospitals in the United Kingdom were no longer carrying out leucotomies.