A number of candidate genes have been reported to be associated with intelligence differences, but the effect sizes have been small and almost none of the findings have been replicated. No individual genetic variants have been conclusively linked to intelligence in the normal range so far. Many researchers believe that very large samples will be needed to reliably detect individual genetic polymorphisms associated with "g". However, while genes influencing variation in "g" in the normal range have proven difficult to find, many single-gene disorders with mental retardation among their symptoms have been discovered.