This paper examines the pattern of diffusion in the academic literatures of the sciences, social sciences, and arts and humanities based on citations. An examination of the citations of articles in the Science Citation Index, Social Science Citation Index, and Arts and Humanities Citation Index from a given year to the year in which the cited article was published reveals a pattern. The percentage is initially small when there is no lag between the years; it then increases, reaching a peak in less than two years. Then, it gradually decreases over time. A mathematical model was developed to describe this pattern, which, when tested, explains between 96.9% and 98.3% of the variance, depending on the data set. The results, which are presented in tabular form, are interpreted as an example of social learning and forgetting. Findings are illustrated in five graphs, and references are provided. (Author/KM)