This article reports the findings of a known-groups validity study of the Empathy Assessment Index (EAI), which is a 20-item self-report instrument based on an emerging social cognitive neuroscience definition of empathy. A convenience sample formed 2 groups of study participants: social service providers (n = 197), and a group (n =453) of offender service recipients from community treatment agencies, receiving services for (a) sexual offenses (n = 251), (b) anger management or misdemeanor domestic violence (n = 155), and (c), domestic violence (n = 47). The literature suggests providers and offenders should have significantly different levels of empathy. To test this hypothesis, we compare the group scores on the overall EAI and 4 component scores (affective response, perspective taking, self–other awareness, emotion regulation). Differential item functioning analysis is used to determine whether the EAI items measure the same construct within each group, and highlights 5 items. Results include findings from a multilevel regression analysis, using the 20- and 15-item versions of the EAI. For both versions, controlling for gender, age, education, income, class, and race variables, we find treatment groups have lower average empathy scores than service providers; this difference is statistically significant for 2 of the 3 treatment groups. The emotion regulation and self–other awareness EAI components demonstrate known-groups validity in 1 of the 3 treatment groups. Researchers can likely increase both known-groups and content validity of the EAI by using perception-action items of greater specificity in the affective response component and by adding an affective mentalzing component.