Many factors impact the demand for and supply of oil and natural gas, influence how and where energy companies invest their capital, and determine the manner in which countries compete to attract foreign investment. World oil supply derives from the investment decisions of individual companies, the political decisions of countries in regard to licensing and degree of foreign investment, and a multitude of other variables that influence system dynamics, including price, inventory levels, geopolitics, market psychology and manipulation, OPEC policy, exchange rates, unexpected events, and resource availability.