Table:
|  |  |  | Year Ended May 31, |  |
| --- | --- | --- | --- | --- |
|  |  |  | Percent Change |  |
| (Dollars in millions) | 2019 | Actual | Constant | 2018 |
| Provision for income taxes | $1,185 | -87% | -86% | $8,837 |
| Effective tax rate | 9.7% |  |  | 71.1% |
Paragraphs:
Provision for Income Taxes: Our effective income tax rates for each of the periods presented were the result of the mix of income earned in various tax jurisdictions that apply a broad range of income tax rates. In fiscal 2018, the Tax Act was signed into law. The more significant provisions of the Tax Act as applicable to us are described above under “Impacts of the U.S. Tax Cuts and Jobs Act of 2017”. refer to Note 14 of Notes to Consolidated Financial Statements included elsewhere in this Annual report for a discussion regarding the differences between the effective income tax rates as presented for the periods below and the U.S. federal statutory income tax rates that were in effect during these periods. Future effective income tax rates could be adversely affected by an unfavorable shift of earnings weighted to jurisdictions with higher tax rates, by unfavorable changes in tax laws and regulations, by adverse rulings in tax related litigation, or by shortfalls in stock-based compensation realized by employees relative to stock-based compensation that was recorded for book purposes, among others.
Provision for income taxes decreased in fiscal 2019 relative to fiscal 2018 primarily due to the absence of the initial accounting charges related to the Tax Act that were recorded in fiscal 2018. To a lesser extent, provision for income taxes also decreased in fiscal 2019 due to the net favorable impacts of our final accounting for the Tax Act in fiscal 2019; the net favorable impacts of the Tax Act on our tax profile during fiscal 2019; the favorable impact of a tax benefit arising from an increase in a deferred tax asset associated with a partial realignment of our legal structure in fiscal 2019; and lower income before provision for income taxes in fiscal 2019. These decreases to our provision for income taxes in fiscal 2019 relative to fiscal 2018 were partially offset both by lower excess tax benefits related to stock-based compensation expense in fiscal 2019, and by less favorable changes in net unrecognized tax benefits due to settlements with tax authorities and other events in fiscal 2019 relative to fiscal 2018.
Question:
What are some reasons provided for possible adverse impact on effective income tax rates?
Answer: Future effective income tax rates could be adversely affected by an unfavorable shift of earnings weighted to jurisdictions with higher tax rates, by unfavorable changes in tax laws and regulations, by adverse rulings in tax related litigation, or by shortfalls in stock-based compensation realized by employees relative to stock-based compensation that was recorded for book purposes, among others.