'some subsidies can have adverse effects on the economy, which are most often efficiency losses as a result of relative market prices being distorted. These economic effects are usually expressed by means of a monetary value, e.g. changes in GDP. Furthermore, and perhaps less well known, some subsidies may have negative environmental effects. For instance when subsidies are used to keep prices artificially low, this may encourage overuse or wasteful use of the resources being subsidised. From the viewpoint of society these subsidies are unwanted if the negative effects outweigh the benefits from the subsidies. Estimating the net benefits can be difficult, though, especially because of inadequate knowledge about the linkages between subsidies and the environmental effects and also because environmental effects are often not valued in monetary terms. Subsidies that are both harmful to the environment and the economy are here referred to as “perverse subsidies”.' (Source: 'Environmentally Harmful Subsidies', Environmental Assessment Institute, 2005)