(in thousands)	Three months ended
	December 31, 2020			December 28, 2019
Income before income taxes	$	77,407		$	19,031
Provision (benefit) for income taxes	$	53,892		$	(16,424	)
Effective income tax rate		70	%		(86	)%

In the first quarter of 2021 and 2020, our effective tax rate differed from the statutory federal income tax rate of 21% due to U.S. tax reform, our corporate structure in which our foreign taxes are at a net effective tax rate lower than the U.S. rate, and the excess tax benefit related to stock-based compensation. A significant amount of our foreign earnings is generated by our subsidiaries organized in Ireland. In 2021 and 2020, the foreign rate differential predominantly relates to these Irish earnings. In addition, the effective tax rate was impacted by the matters described below.

Our first quarter of 2021 results include a charge of $35.3 million related to the effects of a tax matter in the Republic of Korea (South Korea) of $32.4 million, and the resulting impact on U.S. income taxes of $2.9 million. The charge relates to an assessment with respect to various tax issues, primarily foreign withholding taxes, under appeal in South Korea. We received an assessment of approximately $12 million from the tax authorities in South Korea in the fourth quarter of 2016 for the years 2011 to 2015 and paid the assessment in the first quarter of 2017. We appealed that assessment and believed that upon completion of the multi-level appeal process it was more likely than not that our positions would be sustained. However, in December 2020, our appeal to the Seoul High Court (an intermediate appellate court) was rejected. We have appealed this decision to the Supreme Court of the Republic of Korea. We continue to believe that our position is meritorious, and we will aggressively pursue our position with the Supreme Court.

In the first quarter of 2020, we reduced our previously established U.S. valuation allowance by $21.0 million as the result of the Onshape acquisition.

We have concluded, based on the weight of available evidence, that a full valuation allowance continues to be required against our U.S. net deferred tax assets as they are not more likely than not to be realized in the future. However, we believe that there is a reasonable possibility that within the next 12 months, sufficient positive evidence may become available to allow us to reach a conclusion that a significant portion of the valuation allowance will no longer be needed. Release of the valuation allowance would result in the recognition of certain deferred tax assets and a decrease to income tax expense for the period the release is recorded. However, the exact timing and amount of any valuation allowance release are subject to change on the basis of the level of profitability that we are able to actually achieve. We will continue to reassess our valuation allowance requirements each financial reporting period.

In the normal course of business, PTC and its subsidiaries are examined by various taxing authorities, including the Internal Revenue Service in the U.S. We regularly assess the likelihood of additional assessments by tax authorities and provide for these matters as appropriate. We are currently under audit by tax authorities in several jurisdictions. Audits by tax authorities typically involve examination of the deductibility of certain permanent items, limitations on net operating losses and tax credits.

As of December 31, 2020 and September 30, 2020, we had unrecognized tax benefits of $46.3 million and $16.1 million, respectively. If all our unrecognized tax benefits as of December 31, 2020 were to become recognizable in the future, we would record a benefit to the income tax provision of $46.3 million, which would be partially offset by an increase in the U.S. valuation allowance of $7.9 million.

Although we believe our tax estimates are appropriate, the final determination of tax audits and any related litigation could result in favorable or unfavorable changes in our estimates. We believe it is reasonably possible that within the next 12 months the amount of unrecognized tax benefits related to the resolution of multi-jurisdictional tax positions could be reduced by up to $30 million.