Conation, or the ability to apply one's abilities purposefully, persistently and effectively as required by the task at hand, has been considered historically to be an important element of psychology, but has been relatively neglected in clinical neuropsychology. Nevertheless, conation may well be a significant missing variable in predicting a person's success in practical situations based on neuropsychological test scores. This study listed 19 tests from the Halstead-Reitan battery in order of their predicted dependence on conation. Differences between a brain-damaged group and a control group were determined through computation of t-ratios. The rank-ordering of the 19 tests according to their requirement of conation and the magnitude of the t-ratios were closely correlated, yielding a rank-difference coefficient of 0.84. These results suggest that conation is a strong factor in determining differential performances on neuropsychological tests. Further research is required to identify the role of conation in determining the ecological validity of neuropsychological test results.