Safety’s outstanding common shares. Under the program, Safety may repurchase shares of its common stock for cash in public or private transactions, in the open market or otherwise, at management’s discretion. The timing of such repurchases and actual number of shares repurchased will depend on a variety of factors including price, market conditions and applicable regulatory and corporate requirements. The program does not require Safety to repurchase any specific number of shares and may be modified, suspended or terminated at any time without prior notice.
For the quarter ended December 31, 2021, losses and loss adjustment expenses incurred increased by $25.1 million, or 26.3%, to $120.9 million from $95.8 million for the comparable 2020 period. For the year ended December 31, 2021, losses and loss adjustment expenses incurred increased by $57.1 million, or 14.1%, to $461.7 million from $404.6 million for the comparable 2020 period. The 2020 losses and loss adjustment expenses in both periods reflect a decrease in frequency, primarily in our private passenger automobile line of business as a result of the COVID-19 pandemic.
Total prior year favorable development included in the pre-tax results for the quarter ended December 31, 2021 was $12.5 million compared to $20.2 million for the comparable 2020 period. Total prior year favorable development included in the pre-tax results for the year ended December 31, 2021 was $53.7 million compared to $54.8 million for the comparable 2020 period.
Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the quarter ended December 31, 2021 were 62.7%, 33.7%, and 96.4%, respectively, compared to 48.8%, 36.3%, and 85.1%, respectively, for the comparable 2020 period. Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the year ended December 31, 2021 were 59.6%, 33.4%, and 93.0%, respectively, compared to 52.5%, 34.6%, and 87.1%, respectively, for the comparable 2020 period. The 2021 decrease in the expense ratios in both 2021 periods is driven by a decrease in contingent commission expense.
Direct written premiums for the quarter ended December 31, 2021 decreased by $1.0 million, or 0.6%, to $181.6 million from $182.6 million for the comparable 2020 period. Direct written premiums for the year ended December 31, 2021 increased by $3.4 million, or 0.4%, to $802.1 million from $798.7 million for the comparable 2020 period. The year ended December 31, 2020 direct written premium reflects the Safety Personal Auto Relief Credit, a 15% policyholder credit that was applied to personal auto policies for the months of April, May and June 2020 as well as changes made by CAR to eligibility requirements which impacted the number of commercial automobile policies that we handle as a Servicing Carrier to the ceded pool. This results in a commensurate decrease in ceded written premium to and assumed from these programs.
Net written premiums for the quarter ended December 31, 2021 decreased by $0.1 million, or 0.1%, to $170.9 million from $171.0 million for the comparable 2020 period. Net written premiums for the year ended December 31, 2021 increased by $1.0 million, or 0.1%, to $764.5 million from $763.5 million for the comparable 2020 period. Net earned premiums for the quarter ended December 31, 2021 decreased by $3.6 million, or 1.9%, to $192.8 million from $196.4 million for the comparable 2020 period. Net earned premiums for the year ended December 31, 2021 increased by $3.3 million, or 0.4%, to $774.3 million from $771.0 million for the comparable 2020 period. The changes in both periods are a result of the changes in direct written premiums as described above.
Net investment income for the quarter ended December 31, 2021 increased by $1.0 million, or 9.5%, to $11.7 million from $10.7 million for the comparable 2020 period. Net investment income for the year ended December 31, 2021 increased by $3.1 million, or 7.5%, to $44.1 million from $41.0 million for the comparable 2020 period. The increase in both periods is a result of an increase in the average invested asset balance and an increase in the equity in earnings of other invested assets compared to the prior year. Net effective annualized yield on the investment portfolio for the quarter ended December 31, 2021 was 3.1% compared to 3.0% for the comparable 2020 period. Net effective annualized yield on the investment portfolio for the year ended December 31, 2021 was 3.0% compared to 2.9% for the comparable 2020 period. Our duration on fixed maturities was 3.6 years at December 31, 2021 compared to 3.2 years at December 31, 2020.