General and administrative expenses. General and administrative expenses for the six months ended June 30, 2021 were $64.1 million, a decrease of $1.3 million, or 1.9%, from the same period in the prior year. The decrease is the result of incremental share-based compensation expense of $5.9 million recognized during the comparative period of 2020 due to the modification of certain equity awards granted in 2017, a decrease in professional accounting and legal fees of $1.0 million, and a decrease in incentive compensation and other personnel-related costs of $0.4 million, offset by increases in salary expense of $4.3 million, and other expenses of $1.7 million compared to the six months ended June 30, 2020.
Depreciation and amortization. Depreciation and amortization for the six months ended June 30, 2021 was $16.0 million, a decrease of $1.7 million, or 9.6%, from the same period in the prior year. Depreciation expense decreased $1.0 million and amortization expense decreased $0.7 million when compared to the same period in the prior year.
Interest expense, net. Interest expense for the six months ended June 30, 2021 decreased 14.3% to $14.5 million from $16.9 million for the same period in the prior year.
Provision (benefit) for income taxes. The provision for income taxes for the six months ended June 30, 2021 was $0.5 million, or 6.3% of income before income taxes, compared to a benefit of $2.8 million, or (22.8)% of income before income taxes for the six months ended June 30, 2020. The effective tax rate for the six months ended June 30, 2021 consisted principally of the 21% federal statutory tax rate, research and development credits, permanent tax differences, and a windfall from share-based compensation. The increase in the effective tax rate for the six months ended June 30, 2021 compared with the six months ended June 30, 2020 is primarily attributable to 2017 through 2019 research and development tax benefits recognized in the six months ended June 30, 2020, partially offset by a windfall from share-based compensation for the six months ended June 30, 2021.
We evaluate our deferred tax assets quarterly to determine whether adjustments to the valuation allowance are appropriate in light of changes in facts or circumstances, such as changes in expected future pre-tax earnings, tax law, interactions with taxing authorities, and developments in case law. Our material assumptions include forecasts of future pre-tax earnings and the nature and timing of future deductions and income represented by the deferred tax assets and liabilities, all of which involve the exercise of significant judgment. As of June 30, 2021, our valuation allowance approximated $2.1 million.
For the year ending December 31, 2021, we estimate a research and development tax credit of $3.0 million, net of tax reserves. We record the tax benefit, net of tax reserves, as a deferred tax asset. For the year ended December 31, 2020, we recognized research and development tax credits of $2.2 million, net of tax reserves, related to 2020, and $6.1 million, net of tax reserves, related to prior years.
Financial Condition, Liquidity, and Capital Resources
Liquidity
Our cash and cash equivalents, and any amounts we have available for borrowing under our revolving credit facility, are immediately available to provide cash for our operations and capital expenditures. We refer to the sum of these two amounts as our “liquidity.”
At June 30, 2021, we had total liquidity of $171.1 million, which reflected a decrease of $68.3 million from the $239.4 million in liquidity we had as of December 31, 2020. Our liquidity at June 30, 2021 was comprised of cash and cash equivalents of $76.2 million and $94.9 million in available borrowing capacity under our $100.0 million revolving credit facility. This decrease in liquidity primarily related to a decrease in cash of $68.4 million, comprised of net cash used in operations of $9.3 million, cash paid for acquisitions, net of cash acquired, of $35.3 million, capital expenditures of $14.2 million, and net cash used in financing activities of $10.9 million.
Our Credit Agreement contains customary representations and warranties, as well as financial covenants, including that we maintain compliance with certain leverage and interest coverage ratios. If we are not compliant with our debt covenants in any period, absent a waiver or amendment of our Credit Agreement, we may be unable to access funds under our revolving credit facility. Due to the additional borrowings under our revolving credit facility in March 2020, which were repaid in full during the third quarter of 2020, and in anticipation of the potential economic impact of the COVID-19 pandemic, we entered into an amendment to the Credit Agreement that provided for, among other things, increases in the allowable level of indebtedness we may carry relative to our earnings, changes in the definition of EBITDA used to compute certain financial ratios, certain restrictions regarding investments and payments we made until the completion of the first quarter of 2021 and increases in the interest costs associated with borrowings under our revolving credit facility. We were in compliance with our debt covenants as of June 30, 2021.