The introduction of a production function of technology embodying laws of returns to research and development (R&D) is now standard practice in growth theory. This paper offers a critical evaluation, in the light of a generalized N–K model, of some recent contributions suggesting foundations for the existence of laws of returns to R&D. It is argued that such contributions fail to analyze the way in which research and development activity in the technological and scientific domains affect the dimension, the hierarchic structure and the complexity of knowledge search spaces. In the attempt at moving some analytical steps in this direction, this paper considers the possibility that modularity effectively counters the rise in complexity which would follow from idea growth and the increasing number of potential interactions between component ideas. It is argued that the force of the modularity argument finds its limits in the face of radical innovations that are general purpose, but entail a deconstruction and reconstruction of the hierarchy of technological interactions. It is also suggested that niche creation and knowledge spillovers elicit the early development and subsequent diffusion of such radical innovations.