written premiums are a result of new business production, improved retention, and rate increases. For the nine months ended September 30, 2023, the Company achieved policy count growth across all lines of business, including 14.1%, 5.9% and 9.7% in Private Passenger Automobile, Commercial Automobile and Homeowners lines, respectively, compared to the same period in 2022. Additionally, for the nine months ended September 30, 2023, average written premium per policy increased 10.7%, 4.5% and 4.5% in Private Passenger Automobile, Commercial Automobile and Homeowners lines, respectively, compared to the same period in 2022.
Net earned premiums for the quarter ended September 30, 2023 increased by $24.5 million, or 12.9%, to $214.4 million from $189.9 million for the comparable 2022 period. Net earned premiums for the nine months ended September 30, 2023 increased by $43.0 million, or 7.6%, to $608.4 million from $565.4 million for the comparable 2022 period.
For the quarter ended September 30, 2023, losses and loss adjustment expenses incurred increased by $35.4 million, or 28.6%, to $159.5 million from $124.1 million for the comparable 2022 period. For the nine months ended September 30, 2023, losses and loss adjustment expenses incurred increased by $110.2 million, or 30.6%, to $470.2 million from $360.0 million for the comparable 2022 period. The increase in losses is driven by current market conditions, specifically inflation, as well as weather events including multiple flood events, a high wind event, and a severe winter weather event that occurred in February.
Loss, expense, and combined ratios calculated for the quarter ended September 30, 2023, were 74.4%, 30.4%, and 104.8%, respectively, compared to 65.3%, 31.8%, and 97.1%, respectively, for the comparable 2022 period. Loss, expense, and combined ratios calculated for the nine months ended September 30, 2023 were 77.3%, 30.9%, and 108.2%, respectively, compared to 63.7%, 32.3%, and 96.0%, respectively, for the comparable 2022 period. The decrease in the expense ratio is primarily driven by a decrease in contingent commission expenses.
Total prior year favorable development included in the pre-tax results for the quarter ended September 30, 2023 was $13.5 million compared to $14.0 million for the comparable 2022 period. Total prior year favorable development included pre-tax results for the nine months ended September 30, 2023 was $35.0 million compared to $43.2 million for the comparable 2022 period, which included the reversal of a $6.5 million accrued reserve for legal defense costs associated with business interruption claims resulting from the COVID-19 pandemic.
Net investment income for the quarter ended September 30, 2023 increased by $2.9 million, or 26.0% to $14.0 million from $11.1 million for the comparable 2022 period. Net investment income for the nine months ended September 30, 2023 increased by $8.2 million, or 24.5%, to $41.5 million from $33.3 million for the comparable 2022 period. The increase is a result of increases in interest rates on our fixed maturity portfolio compared to the prior year. Net effective annualized yield on the investment portfolio was 4.0% for the quarter ended September 30, 2023 compared to 3.1% for the comparable 2022 period. Net effective annualized yield on the investment portfolio for the nine months ended September 30, 2023 was 3.9% compared to 3.0% for the comparable 2022 period. Our duration on fixed maturities was 3.6 years at September 30, 2023 compared to 3.8 years at December 31, 2022.
Non-GAAP Measures
Management has included certain non-GAAP financial measures in presenting the Company’s results. Management believes that these non-GAAP measures are useful to 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.