Recently, there has been much work on the importance of diversity, particularly species diversity, on process and function. Evidence indicates that diversity may be important at many different levels of organization – among functional types, among species, and among individuals. In all cases, theory predicts that the strength of intra-group competition is stronger than inter-group competition, driven by trait differences among groups that allow partitioning and more efficient use of heterogeneous resources. We conducted a meta-analysis to assess whether this prediction applies to genetic diversity. We quantitatively reviewed literature that examined the effects of intraspecific competition on fitness (direct and indirect measures) between populations comprised of genetically-similar and genetically-dissimilar individuals. We also assessed effects on measures of population function, such as resilience to disturbance. We predicted that increasing the genotypic diversity (genetic dissimilarity) would positively affect measures of both fitness and function. In a survey of work published between 1975 and 2006, we found 24 studies for 41 appropriate comparisons among plant, animal, and bacterial species. Across the studies, there was no effect of increasing genotypic diversity on measures of fitness. Several studies found a sampling effect, in which one genotype or family out-performed all others. Across the studies that measured effects of diversity on population function, there was a significant positive effect of increased genotypic diversity. Thus, while increases in genotypic diversity did not reduce the intensity of intraspecific competition, the presence of specific genotypes often had positive effects on population growth or fitness. In addition, the positive effects of increased genotypic diversity may manifest themselves in measures of population function, perhaps analogous to diversity effects on ecosystem function.