This article examines the analysis of the psychometric properties, including the validity and reliability, of the Empathy Index (EI), a new instrument designed to measure empathy deficits of sex offenders. The EI was tested with a sample of 158 sex offenders incarcerated in North Carolina prisons. An exploratory factor analysis yielded three subscales: social aggression; instrumental (proactive) aggression; and justification. Social aggression was an unexpected finding, given this type of aggression is more commonly found in young females rather than adult male sex offenders. The instrument demonstrates promising construct and concurrent validity as well as strong internal reliability. Implications for future research and practice are discussed. (Contains 4 tables.)