This contribution presents a model of morphological productivity that allows some points of comparison with the concept of productivity in economics and in other disciplines. It is a two-step model: in the first step, degrees of productivity of morphological rules are established on the so-called syntagmatic axis of language, i.e. how a word is changed in its linear make-up, e.g. English boy becomes boy-s in the plural or is changed to boy-friend, boy-hood, boy-ish in word formation. In the second step, rule application is compared on the so-called paradigmatic axis with possibly competing productive rules, e.g. in adjective formation from nouns, suffixation with -ish (e.g. boy-ish), is compared with suffixations with -y (e.g. fier-y), -ly (e.g. friend-ly), -ic (e.g. syntact-ic), -ical (e.g. morpholog-ical). Then supportive psycholinguistic evidence is presented from online tests and first language acquisition, an area of crucial evidence for linguistic theories, since any constructs of a model must be learnable by children.