Two components of a causal explanation of Bradford's law

One can suppose that Bradford's law is valid for all scientific fields. As an implication of this general validity and because of limitations of space, journals must differ in their subject structure, and every journal must have its own hierarchy of subjects, conforming to a Bradford or a similar distribution. The phenomenon of subject hierarchies is shown here for ten journals in twentieth-century psychology and mathematical logic and for five journals in nineteenth-century mathemat ics, taking instead of Bradford's original rank-size distribu tion the equivalent, but more general, Pareto distribution. It is hypothesised then that hierarchies of subjects within jour nals correspond to the reception process, i.e. to the structure of interests of their readers. This is illustrated by means of an example of 30 most prolific nineteenth-century mathemati cians. It is argued that the phenomenon of subject hierarchies in journals and in readers has to be considered in a causal explanation of Bradford's law.