The law of diminishing returns is central to the discipline of economics. But it is a lesson that reformers in the Harvard department appear not yet to have learned--and traditionalists seem determined to teach them.

For at least the past five years reformers have sought to modify the department's generally conservative bias, first by demanding that the ranks of tenured professors include Marxian economists, then, in the past year, by working within the department's committee structure to insure a place in the curriculum where Marxian economics could be systematically taught.

But no matter the approach or the intensity of the effort, it seems that the department remains consistently resistant to admitting Marxian analysis or Marxians into its teaching ranks or its graduate curriculum.

The rejection Tuesday of the recommendations of a committee chaired by Kenneth T. Arrow, professor of Economics, adds more data to the axiom's proof.

For all the political activity focused on the issue, the number of Marxians teaching in the department has shrunk from a high of five years ago to a distinct possibility that there may be none at all teaching next year.

The Arrow Committee, chaired by a Nobel-prize winning department liberal, urged that Marxian analysis be part of the economic theory course required of graduate students and that at least two instructors be hired to teach such graduate courses.

The committee avoided the touchy question of tenure for radical economists, over which past battles in the department have been centered. It sought to outline a moderate, structural way in which Marxian economics could find its way gradually into the department's mainstream.

Instead the department voted to allow graduate students to use a course in Marxian economic history to meet part of their history requirement, included some more radical readings on its theory syllabus, and gave first priority to the hiring of a Marxian assistant professor next year--pretty weak medicine, the reformers said.

Another old law states that if one doesn't get the full loaf, settle for half a loaf. The reformers learned and obeyed that law, but all they say they have gotten from the department was a few crumbs.