Preoperative psychological screening of bariatric surgery candidates has become routine, and a significant proportion of patients have their surgery deferred as a consequence. If psychological testing is being used as a form of preoperative triage, both patients and surgeons are entitled to know whether there is sufficient evidence to justify its use in this way. We define the argument for psychological screening as consisting of four premises (p1-p4) and a conclusion (C) as follows: (p1) A significant minority of obese patients will not be successful in losing weight following bariatric surgery-the "failure" group; (p2) A significant minority of patients will exhibit abnormal psychological profiles during preoperative testing; (p3) The majority of individuals referred to in (p2) will be found in group (p1) i.e., abnormal psychological profiles identified preoperatively predict less favorable weight loss outcomes postoperatively; (p4) Identifying patients with adverse psychological profiles preoperatively would allow either exclusion of those at high risk of failure or provide a more secure rationale for targeted pre- and postoperative support; (C) Psychological screening should be part of the routine preoperative assessment for patients undergoing obesity surgery. We reviewed the literature to find evidence to support the premises and show that (p1) can be justified but that (p2) is problematic and can only be accepted in a heavily qualified version. We find no evidence for (p3) and since (p4) and (C) are predicated on (p3), the argument clearly fails. There is no evidence to suggest that preoperative psychological screening can predict postoperative outcomes and no justification for using such testing as a means of discriminating between candidates presenting themselves for bariatric surgery.