I model research quality as the outcome of a CES production technology that uses human capital measured by publication records as inputs. Investigating a sample of scientific publications with two co-authors I show that the CES-complementarity parameter is a function of the age difference of the authors. Complementarity is maximized if the age difference between the authors is about 10 years. Two theories are presented which may explain my findings. According to these models, older and younger researchers differ not only in their skill levels but also in the types of their skills and their interpersonal relationships.