Vis-à-vis tax revenues, much less academic study has been conducted into the volume and distribution of non-tax revenues, although the most significant forms — oil and natural gas revenues and foreign aid — have been extensively studied since Hossein Mahdavy’s seminal 1970 analysis of the
Imperial State of Iran. In 2009, Farhan Zainulabideen and Zafar Iqbal estimated non-tax revenues to comprise a quarter of total global
government revenue. Three years later, Christian von Haldenwang and Maksym Ivanyna produced a higher estimate of around 31 percent. Twenty-first century studies show that non-tax revenue in
petrostates can reach up to 80 percent of
Gross Domestic Product and over 90 percent of total government revenue. In resource-poor nations — excluding those gaining strategic rents due to geography or perceived need for aid — non-tax revenues are typically around 10 percent of total government revenue.
Volatility Non-tax revenues fluctuate much more from one year to another than taxes — three times as much in the
European Union, and slightly less than that for the globe as a whole. Many countries in Africa can report changes in non-tax revenue of over 35 percent from one year to another due to variations in the price of their natural resources. Their value is correlated with changing economic circumstances, repayments and interest on loans may be renegotiated, a record fine in the field of competition can significantly vary the profits of fines and penalties. Moreover, some years are marked by exceptional events: for example, in France in 2012, the sale of "4G" radio frequencies resulted in the collection of nearly €1.3 billion in non-tax revenues. ==Effects==