This paper analyzes Scholastic Aptitude Test (SAT) score declines of two decades--the 1960s and 1970s. The importance of pervasive societal influences is accepted. A variety of data suggests, however, that the increase in school retention rates of poorly prepared students and the resulting heterogeneity of the high school population is a unifying exploratory variable for the second leg of the decline as well as the first. The argument is that several of the "pervasive influences" invoked to explain the continuation of the decline in the 1970s are best understood as adaptive responses of the schools to the appearance of a greater diversity of students in senior high schools. These responses, which in combination represent a reduction in the demand level of the school program, included grade inflation, proliferation of electives, textbook simplification, and reduction in homework assignments. Students also chose fewer academic and more vocational and general courses. It is hypothesized that the continuation of the decline in the 1970s was, in substantial part, the direct consequence of those school-related changes and thus was a delayed, indirect consequence of the compositional shift. (Author/LMO)