$589.3 million for the comparable 2019 period. The decreases in both periods are a result of the decrease in direct written premiums as described above.
The pandemic has resulted in fewer cars on the road, resulting in a decrease in frequency of claims, primarily in our private passenger automobile line of business. As a result, for the quarter ended September 30, 2020, loss and loss adjustment expenses incurred decreased by $37.9 million, or 28.1%, to $97.1 million from $135.0 million for the comparable 2019 period. For the nine months ended September 30, 2020, loss and loss adjustment expenses incurred decreased by $74.6 million, or 19.5%, to $308.8 million from $383.4 million for the comparable 2019 period. Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the quarter ended September 30, 2020 were 49.8%, 35.2%, and 85.0%, respectively, compared to 68.0%, 30.7%, and 98.7%, respectively, for the comparable 2019 period. Loss, expense, and combined ratios calculated under U.S. generally accepted accounting principles for the nine months ended September 30, 2020 were 53.7%, 34.0%, and 87.7%, respectively, compared to 65.1%, 30.9%, and 96.0%, respectively, for the comparable 2019 period. Total prior year favorable development included in the pre-tax results for the quarter ended September 30, 2020 was $15.3 million compared to $3.2 million for the comparable 2019 period. The prior year favorable development for the three months ended September 30, 2019 reflects adverse development on a settlement involving a liability claim that sought to recover extra-contractual damages against one of our personal umbrella policyholders. Total prior year favorable development included in the pre-tax results for the nine months ended September 30, 2020 was $34.6 million compared to $25.5 million for the comparable 2019 period.
The increase in the expense ratios in the respective periods is driven by an increase in contingent commission expense as well as costs associated with various system modernization in our claims, billing and underwriting areas and a reduction in certain expense allowances offered under the Servicing Carrier program that have decreased with the related written premium as noted above.
Net investment income for the quarter ended September 30, 2020 decreased by $2.2 million, or 18.7%, to $9.7 million from $11.9 million for the comparable 2019 period. Net investment income for the nine months ended September 30, 2020 decreased by $4.0 million, or 11.5%, to $30.3 million from $34.3 million for the comparable 2019 period. The decreases in both periods is a result of lower floating yields on our bank loan portfolio, lower interest income on our partnership investments and 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 September 30, 2020 was 2.8% compared to 3.5% for the comparable 2019 period. Net effective annualized yield on the investment portfolio for the nine months ended September 30, 2020 was 2.9% compared to 3.4% for the comparable 2019 period. Our duration on fixed maturities was 3.2 years at September 30, 2020 compared to 3.3 years at December 31, 2019.
On November 4, 2020, our Board of Directors approved a $0.90 per share quarterly cash dividend on its issued and outstanding common stock payable on December 15, 2020 to shareholders of record at the close of business on December 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.
Non-GAAP operating income and non-GAAP operating income per diluted share consist of our GAAP net income adjusted by the net realized gains (losses) on investments, net impairment losses on investments, change in net unrealized gains on equity investments, credit loss benefit (expense) and taxes related thereto. For the quarter ended September 30, 2020, an increase of $7.5 million for the change in unrealized gains on