The concept of attitudes has been used and investigated to predict human behaviors for about a century. Review of researches on the definition of attitudes shows that attitudes have been regarded as "evaluation toward objects" at least in the empirical researches on attitudes. There are many empirical researches on formation and changing of evaluation toward objects which is assumed to be a predictor of human behaviors. In this article, they are reviewed and divided into three categories, which are researches based on the reinforcement theory, researches based on the cognitive theory, and researches with mathematical models. Finally, it is proposed that studies of human information processing are applicable to attitude studies, and many cognitive elements have to be used to clarify attitude structure and to predict human behaviors.
