June 30, 2020 decreased by $20.6 million, or 5.0%, to $386.6 million from $407.2 million for the comparable 2019 period. Net earned premiums for the quarter ended June 30, 2020 decreased by $14.5 million, or 7.4%, to $181.9 million from $196.4 million for the comparable 2019 period. Net earned premiums for the six months ended June 30, 2020 decreased by $11.2 million, or 2.8%, to $379.7 million from $390.9 million for the comparable 2019 period. The decreases in both periods are a result of the premium relief as described above.
The pandemic has resulted in fewer cars on the road, resulting in a decrease in frequency, primarily in our private passenger automobile line of business. As a result, for the quarter ended June 30, 2020, loss and loss adjustment expenses incurred decreased by $31.4 million, or 25.7%, to $91.0 million from $122.4 million for the comparable 2019 period. For the six months ended June 30, 2020, loss and loss adjustment expenses incurred decreased by $36.7 million, or 14.8%, to $211.7 million from $248.4 million for the comparable 2019 period. Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the quarter ended June 30, 2020 were 50.0%, 34.9%, and 84.9%, respectively, compared to 62.3%, 31.0%, and 93.3%, respectively, for the comparable 2019 period. Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the six months ended June 30, 2020 were 55.7%, 33.3%, and 89.0%, respectively, compared to 63.5%, 31.0%, and 94.5%, respectively, for the comparable 2019 period. Total prior year favorable development included in the pre-tax results for the quarter ended June 30, 2020 was $9.7 million compared to $10.4 million for the comparable 2019 period. Total prior year favorable development included in the pre-tax results for the six months ended June 30, 2020 was $19.3 million compared to $22.3 million for the comparable 2019 period.
The increase in the expense ratios in the respective periods is partially driven by costs associated with various system modernization in our claims, billing and underwriting areas, a reduction in certain expense allowances offered under the Servicing Carrier program that have decreased with the related written premium as noted above and an increase in contingent commission expense. Furthermore, the loss, expense and combined ratios in both periods reflect a lower earned premium base in their respective ratio calculations due to the premium relief efforts noted above.
Net investment income for the quarter ended June 30, 2020 decreased by $0.7 million, or 6.2%, to $9.9 million from $10.6 million for the comparable 2019 period. Net investment income for the six months ended June 30, 2020 decreased by $1.7 million, or 7.6%, to $20.6 million from $22.3 million for the comparable 2019 period. The decrease in net investment income for the quarter ended June 30, 2020 is a result of lower yields on our fixed maturities, specifically bank loans, as well as a decrease in interest income on our partnership investments. The decrease for the six months ended June 30, 2020 is a result of fixed maturity amortization resulting from prepayment activity on certain residential mortgage-backed securities. Net effective annualized yield on the investment portfolio for the quarter ended June 30, 2020 was 2.9% compared to 3.2% for the comparable 2019 period. Net effective annualized yield on the investment portfolio for the six months ended June 30, 2020 was 3.0% compared to 3.3% for the comparable 2019 period. Our duration on fixed maturities was 3.3 years at June 30, 2020 and December 31, 2019.
On August 5, 2020, our Board of Directors approved a $0.90 per share quarterly cash dividend on its issued and outstanding common stock payable on September 15, 2020 to shareholders of record at the close of business on September 1, 2020.
Non-GAAP Measures
Management has included certain non-GAAP financial measures in presenting the Company’s results. Management believes that these non-GAAP measures better explain the Company’s results of operations and allow for a more complete understanding of the underlying trends in the Company’s business. These measures should not be viewed as a substitute for those determined in accordance with generally accepted accounting principles (“GAAP”). In addition, our definitions of these items may not be comparable to the definitions used by other companies.