adjustment expenses for the six months ended June 30, 2021 was 46.4% compared to 46.7% for the comparable 2020 period. Total prior year favorable development included in the pre-tax results for the three months ended June 30, 2021 was $13,349 compared to $9,744 for the comparable 2020 period. Total prior year favorable development included in the pre-tax results for the six months ended June 30, 2021 was $25,808 compared to $19,328 for the comparable 2020 period.
Underwriting, Operating and Related Expenses. Underwriting, operating and related expenses for the three months ended June 30, 2021 increased by $1,575, or 2.5%, to $65,089 from $63,514 for the comparable 2020 period. Underwriting, operating and related expenses for the six months ended June 30, 2021 increased by $3,517, or 2.8%, to $130,113 from $126,596 for the comparable 2020 period. Our GAAP expense ratio for the three months ended June 30, 2021 decreased to 33.5% from 34.9% for the comparable 2020 period. Our GAAP expense ratio for the six months ended June 30, 2021 increased to 33.6% from 33.3% for the comparable 2020 period.
Interest Expense. Interest expense was $130 for the three months ended June 30, 2021 and 2020. Interest expense was $259 for the six months ended June 30, 2021 compared to $177 for the comparable 2020 period. The credit facility commitment fee included in interest expense was $37 for the six months ended June 30, 2021 and 2020.
Income Tax Expense. Our effective tax rate was 20.7% and 20.1% for the three months ended June 30, 2021 and 2020, respectively. Our effective tax rate was 20.4% and 19.5% for the six months ended June 30, 2021 and 2020. The effective tax rates for the three and six months ended June 30, 2021 and 2020 were lower than the statutory rate primarily due to the effects of tax-exempt investment income and the impact of stock-based compensation.
Net Income. Net income for the three months ended June 30, 2021 was $37,667 compared to net income of $42,494 for the comparable 2020 period. Net income for the six months ended June 30, 2021 was $73,841 compared to net income of $40,504 for the comparable 2020 period.
Non-GAAP Operating Income. Non-GAAP operating income, as defined above, was $27,987 for the three months ended June 30, 2021 compared to $29,739 for the comparable 2020 period. Non-GAAP operating income was $56,843 for the six months ended June 30, 2021 compared to $53,921 for the comparable 2020 period.
Liquidity and Capital Resources
As discussed in the Regulatory Matters section below, as a holding company, Safety’s assets consist primarily of the stock of our direct and indirect subsidiaries. Our principal source of funds to meet our obligations and pay dividends to shareholders, therefore, is dividends and other permitted payments from our subsidiaries, principally Safety Insurance. Safety is the borrower under our credit facility.
Safety Insurance’s sources of funds primarily include premiums received, investment income, and proceeds from sales and redemptions of investments. Safety Insurance’s principal uses of cash are the payment of claims, operating expenses and taxes, the purchase of investments, and the payment of dividends to Safety.
Net cash provided by operating activities was $62,590 and $25,229 during the six months ended 2021 and 2020, respectively. Our operations typically generate positive cash flows from operations as most premiums are received in advance of the time when claim and benefit payments are required. Positive operating cash flows are expected in the future to meet our liquidity requirements.
Net cash used for investing activities was $6,715 and $4,890 during the six months ended June 30, 2021 and 2020, respectively. Fixed maturities, equity securities, and other invested assets purchased were $192,441 for the six months ended June 30, 2021 compared to $119,074 for the comparable prior year period. Proceeds from maturities, redemptions, calls and sales, of securities were $191,151 during the six months ended June 30, 2021 compared to $119,510 for the comparable prior year period.