evidence: Legislation passed over the past decade has changed this. The Scottish and Welsh Parliaments have taken full control of some smaller taxes, and have been granted power over aspects of the income tax system. These reforms were designed to improve the financial accountability of devolved administrations, and to incentivise them to choose policies that stimulate growth in their tax base. [...] Since 1999, tax revenue in Wales and Northern Ireland has been significantly lower than in England, while that of Scotland has been more volatile, principally due to fluctuating offshore oil and gas revenue. [...] Tax devolution is accounted for by adjustments to the block grants that the Treasury provides to fund devolved public services. Roughly speaking, when devolved tax revenue rises faster than equivalent revenue elsewhere in the UK, the devolved block grant is increased, and vice versa.