During the 1940s and 1950s, chemists had trouble explaining why even slight changes in the reactants caused significant differences in the rate and product distributions of a reaction.  In 1955 George Hammond, a young professor at Iowa State University, postulated that transition-state theory could be used to qualitatively explain the observed structure-reactivity relationships.  Notably, John E. Leffler of Florida State University proposed a similar idea in 1953. However, Hammonds version has received more attention since its qualitative nature was easier to understand and employ than Lefflers complex mathematical equations. Hammond's postulate is sometimes called the Hammond–Leffler postulate to give credit to both scientists.