This second market failure is due to the fact that firms investing in R&D activities cannot entirely prevent the diffusion of the knowledge gained through these activities to others, including their competitors. Although such "knowledge spillovers" are considered "positive externalities" (i.e. they make the social value of R&D greater than its private value), they legitimise the government support to R&D activities - independently of climate policy. They have developed a unified framework to assess the six different policy options for reducing GHG emissions and promoting the development and diffusion of renewable energy in the electricity sector.