Payroll support grant. In January 2021, we entered into an agreement with U.S. Treasury and received $233.1 million in emergency relief through the 2021 Appropriations Act payroll support program, of which $193.2 million was in the form of payroll support grants that were recognized as a reduction in labor expense over the periods the grants intended to compensate. Additionally, in April 2021, the Company received an additional $35.0 million in proceeds under the PSP Extension Agreement, of which $24.5 million was in the form of payroll support grants that were recognized as a reduction of labor expense during the period the grant was intended to compensate. In April 2021, we also entered into an agreement with U.S. Treasury and received $250.0 million in emergency relief through the American Rescue Plan Act payroll support program, of which $205.0 million was in the form of payroll support grants that are being recognized as a reduction in labor expense over the periods the grants intended to compensate. We recognized $307.3 million in payroll support grant proceeds we received as a reduction to our operating expenses for the six months ended June 30, 2021, compared to $151.9 million in payroll support grant proceeds we received under similar agreements with U.S. Treasury as a reduction to our operating expenses for the six months ended June 30, 2020.
Other operating expenses. Other operating expenses primarily consist of property taxes, hull and liability insurance, simulator costs, crew per diem, crew hotel costs and credit loss reserves. The $5.2 million, or 4.8%, increase in other operating expenses was primarily related to an increase in other operating costs that correspond to the higher number of flights we operated during the six months ended June 30, 2021 compared to the six months ended June 30, 2020, such as crew per diem, crew hotel costs and simulator costs.
Interest Expense. The $4.2 million, or 6.8%, increase in interest expense was primarily related to $3.6 million of deferred loan costs expense attributed to the payoff and termination of the secured loan agreement with U.S. Treasury during the six months ended June 30, 2021 and the additional interest expense associated with four new E175 aircraft added to our fleet subsequent to June 30, 2020, which were debt financed.
Total airline expenses. Our total airline expenses, comprised of our total operating expenses and interest expense, decreased $18.2 million, or 1.7%, due to the payroll support grant benefit we recorded during the six months ended June 30, 2021 compared to the benefit recorded for the six months ended June 30, 2020, offset by an increase in direct operating costs attributed to the increased number of completed flights during the six months ended June 30, 2021 compared to the six months ended June 30, 2020. As our interest expense is primarily attributed to debt associated with financing aircraft under our capacity purchase agreements and as revenue earned under our capacity purchase agreements is intended to compensate us for our aircraft ownership costs, including interest expense, we believe our total airline expense is meaningful expense measure for management discussion and analysis purposes.
Summary of interest income, other income (expense) and provision for income taxes:
Interest income. Interest income decreased $3.8 million, or 88.4%, during the six months ended June 30, 2021, compared to the six months ended June 30, 2020. The decrease in interest income was primarily related to a decrease in average interest rates attributed to our marketable securities subsequent to June 30, 2020.
Other income (expense), net. Other income primarily consisted of income related to our investment in a joint venture with a third party.
Provision for income taxes. For the six months ended June 30, 2021 and 2020, our income tax provision rates were 25.5% and 26.8%, respectively, which include the statutory federal income tax rate of 21% and other reconciling income tax items, including state income taxes and the impact of non-deductible expenses. The decrease in the effective tax rate primarily relates to a $1.4 million discrete tax benefit from excess tax deductions generated from employee equity transactions that occurred during the six months ended June 30, 2020 and a lesser impact related to non-deductible expenses for the six months ended June 30, 2021, compared to the six months ended June 30, 2020 as a result of higher pretax earnings for the six months ended June 30, 2021 compared to the same period of 2020.
Net income. Primarily due to the factors described above, we generated net income of $97.9 million, or $1.93 per diluted share, for the six months ended June 30, 2021, compared to net income of $4.3 million, or $0.08 per diluted share, for the six months ended June 30, 2020.