Such efforts are non-rival since climate mitigation provided to one does not reduce the level of mitigation that anyone else enjoys. They are non-excludable actions as they will have global consequences from which no one can be excluded. A country's incentive to invest in carbon abatement is reduced because it can "free ride" off the efforts of other countries. Over a century ago, Swedish economist Knut Wicksell (1896) first discussed how public goods can be under-provided by the market because people might conceal their preferences for the good, but still enjoy the benefits without paying for them. Valuation Assessing the economic value of the environment is a major topic within the field. The values of natural resources often are not reflected in prices that markets set and, in fact, many of them are available at no monetary charge. This mismatch frequently causes distortions in pricing of natural assets: both overuse of them and underinvestment in them. Economic value or tangible benefits of ecosystem services and, more generally, of natural resources, include both use and indirect (see the nature section of ecological economics). Non-use values include existence, option, and bequest values. For example, some people may value the existence of a diverse set of species, regardless of the effect of the loss of a species on ecosystem services. The existence of these species may have an option value, as there may be the possibility of using it for some human purpose. For example, certain plants may be researched for drugs. Individuals may value the ability to leave a pristine environment for their children. Use and indirect use values can often be inferred from revealed behavior, such as the cost of taking recreational trips or using hedonic methods in which values are estimated based on observed prices. Non-use values are usually estimated using stated preference methods such as contingent valuation or choice modelling. Contingent valuation typically takes the form of surveys in which people are asked how much they would pay to observe and recreate in the environment (willingness to pay) or their willingness to accept (WTA) compensation for the destruction of the environmental good. Hedonic pricing examines the effect the environment has on economic decisions through housing prices, traveling expenses, and payments to visit parks. State subsidy Almost all governments and states magnify environmental harm by providing various types of subsidies that have the effect of paying companies and other economic