Saving for a rainy day gets tricky when piggy banks bulge but no one can agree on the weather. For countries lucky enough to have funds for the safekeeping of oil revenues, high prices have boosted assets to staggering levels. This has led to some bitter arguments about what the money is for and how it should be managed.

Some funds are ring-fenced from governments and have independent, long-term goals – usually the preservation of wealth for future generations. Others are used as short-term cushions for federal budgets, foreign exchange accounts or even as slush funds. Problems arise when mandates are poorly defined.