An economic theory claiming that economic growth is generated from within a system as a direct result of internal processes, underlining the way some regions create an internal mechanism for promoting or perpetuating their growth. Firms operating in places with pools of skilled professionals, for example, are more innovative, leading to faster technological and productivity growth (Ó Huallacháin and Leslie (2007) J. Econ. Geog. 7, 6). Frenken and Boschma (2007) J. Econ. Geog. 7, 5 observe that the more product varieties are already present in a firm or city, the higher the probability that new product varieties can be generated through recombination of old routines—‘this feedback relationship is non-linear in that the potential for new ideas rises more than proportional with the stock of existing ideas’. See also D. Felsenstein et al., eds (2001).