Over recent years there has been increasing interest in personal gain as a threat to the validity of neuropsychological testing. As a result, clinicians have begun to use specialist measures in an attempt to identify how much ‘effort’ a person is putting in and therefore whether they may be feigning or exaggerating their difficulties. Such effort tests have become commonplace within a medico-legal context and their use is also increasing within clinical settings. However, thorough investigation of the performance and classification accuracy of such measures is limited and questions have been raised regarding the rates of false positives in clinical populations. Assessing how people with genuine injuries perform on effort tests is critical for the valid interpretation of test scores, as clinicians have a duty of care not to label people as having a brain injury if they are malingering, or diagnosing someone as malingering when they have a genuine brain injury or are legitimately unwell. Therefore, the current thesis investigated the base rates of failure on a number of effort tests in a genuinely brain-injured population with no identifiable incentives to feign in order to provide further evaluation of the measures. The main focus was on the Word Memory Test, as the author claims that this measure is “virtually insensitive to all but the most extreme forms of impairment of learning and memory” (Green, Lees-Haley & Allen, 2002, p. 99).A total of 47 participants were recruited to the study, including 20 people in residential community rehabilitation services, 16 outpatients with intractable epilepsy, and 11 people in post-acute inpatient rehabilitation. Each participant was administered a battery of tests, including measures of effort, pre-morbid IQ, memory, speed of processing, and mood. Analyses of pass/fail rates across effort tests indicated that the rates of false positives within a genuine clinical sample with no incentive to feign were much higher than those proposed within the validation research of the tests. In addition, further statistical analyses identified a number of factors that contributed to scores on tests in addition to effort. Relationships with these factors varied depending on the particular effort test being assessed, with significant associations being identified with memory, depression, processing speed, age and participant subgroup. These findings are consistent with recent research that suggests people with genuine brain injuries can fail effort tests for reasons related to ability rather than effort. The implications for clinical assessment and intervention are discussed, and potential future research is suggested.