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Background: Aberrant reward behavior is known to associate with various forms of psychopathology. ‘Effort’, as one component of reward, is defined as the perception of the value of the prospective outcome as a function of its magnitude and the perceived costs of the physical or cognitive labor required to obtain it. This study sought to empirically validate the ‘effort’ construct via behavioral assessments and self-reports. Methods: Participants were 1536 children and 1270 of their parents. ‘Effort’ was assessed via the Effort Expenditure for Rewards Task (EEfRT). Child psychopathology was measured using the Child Behavior Checklist; adult psychopathology was measured using the Adult Self Report. Multitrait-multimethod matrices were used to demonstrate the divergent validity of ‘effort’ measures from other measures of reward behavior. We used linear regression models to assess familial transmission of ‘effort’, and zero-inflated negative binomial regression models to analyze the relationship between ‘effort’ and psychopathology and the moderating effects of sex. Results: For adults and their children, measures of ‘effort’ did not correlate with any other measures of reward constructs, suggesting divergent validity. Measures of ‘effort’ were positively correlated between siblings, while correlations between children and adults were not significant. In children, significant interactions were observed between sex and ‘effort’ valuation, in association with anxiety and thought problems, consistent with previous work. The moderating effects of sex had the greatest effect at low ‘effort’, with females having higher severity of psychopathology than males on average. Conclusions: This study identified the ‘effort’ component of reward behavior as a significant correlate of multiple psychopathologies, moderated by sex. These findings provide further evidence supporting ‘effort’ as a relevant construct for understanding psychopathology. The results consistently demonstrate aberrant ‘effort’ is associated with greater anxiety and thought problems. The presence of sibling-sibling correlations accompanied by the absence of parent-child correlations suggests a lack of continuity between child and parent measures and suggests that further evaluation and refinement of age-appropriate measures of effort is warranted.