This study examined the cognitive effects of fraternity/sorority affiliation on 2,293 first-year college students at 18 four-year institutions in 15 states. It assessed the net affects of Greek affiliation on standardized measures of reading comprehension, mathematics, and critical thinking, and sought to determine if the cognitive impacts of Greek affiliation differed for students in different institutional contexts or for students with different characteristics. The study found that Greek-affiliated men had significantly lower end-of-first-year scores on standardized measures of reading comprehension, mathematics, critical thinking, and composite achievement than their non-Greek counterparts. The impact of Greek affiliation on non-white males, however, was slightly positive. Women who joined sororities had lower end-of-first-year scores on all four cognitive measures than non-Greek women, but only the differences in reading comprehension and composite achievement were statistically significant. The findings suggest that the normative peer culture and socially-orientated time commitments of Greek life often are inconsistent with the educational and intellectual mission of colleges and universities. (Contains 50 references.) (MDM)