Other papers highlight different mechanisms leading to internal development of innovation vs. start-ups formation. Lewis and Yao (2001) develop a contracting and matching model in which a firm may optimally choose an open R&D environment, promoting workers' mobility in order to increase its attractiveness as employer. Cassiman and Ueda (2006) consider a dynamic model in which a firm has a limited capacity to commercialize innovations through internal ventures, so that start-ups may emerge as an efficient outcome. Hellmann (2007) investigates the trade-off between focusing (wealth constrained)workers on their assigned tasks vs. giving them adequate incentives for innovative ideas, showing that different equilibria are possible — in which incumbents may discourage innovation, new ideas are developed through internal ventures (spin offs), or employees leave to form a start-up. Klepper and Thompson (2010) develop a theory of spin-offs formation based on strategic disagreement that may lead to the underestimation of good ideas and the creation of new firms. Finally, Hellman and Perotti (2011) highlight that markets and firms complement each other, with firms generating new ideas, and market completing many. A range of organizational structures – from internal ventures to different types of start-ups – may emerge depending on the costs of generating ideas.