Loss and Loss Adjustment Expenses. Loss and loss adjustment expenses incurred for the three months ended September 30, 2020 decreased by $37,912, or 28.1%, to $97,054 from $134,966 for the comparable 2019 period. Loss and loss adjustment expenses incurred for the nine months ended September 30, 2020 decreased by $74,612, or 19.5%, to $308,774 from $383,386 for the comparable 2019 period. The decreases in both periods are a result of a decrease in frequency, primarily in our private passenger automobile line of business.
Our GAAP loss ratio for the three months ended September 30, 2020 decreased to 49.8% from 68.0% for the comparable 2019 period. Our GAAP loss ratio for the nine months ended September 30, 2020 decreased to 53.7% from 65.1% for the comparable 2019 period. Our GAAP loss ratio excluding loss adjustment expenses for the three months ended September 30, 2020 was 38.5% compared to 59.5% for the comparable 2019 period. Our GAAP loss ratio excluding loss adjustment expenses for the nine months ended September 30, 2020 was 43.9% compared to 56.4% for the comparable 2019 period. Total prior year favorable development included in the pre-tax results for the three months ended September 30, 2020 was $15,291 compared to $3,204 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 $34,619 compared to $25,541 for the comparable 2019 period.
Underwriting, Operating and Related Expenses. Underwriting, operating and related expenses for the three months ended September 30, 2020 increased by $7,751, or 12.7%, to $68,596 from $60,845 for the comparable 2019 period. Underwriting, operating and related expenses for the nine months ended September 30, 2020 increased by $13,005, or 7.1%, to $195,192 from $182,187 for the comparable 2019 period. Our GAAP expense ratio for the three months ended September 30, 2020 increased to 35.2% from 30.7% for the comparable 2019 period. Our GAAP expense ratio for the nine months ended September 30, 2020 increased to 34.0% from 30.9% for the comparable 2019 period. The increases in both periods are 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.
Interest Expense. Interest expense was $131 for the three months ended September 30, 2020 compared to $22 for the comparable 2019 period. Interest expense was $308 for the nine months ended September 30, 2020 compared to $67 for the comparable 2019 period. The credit facility commitment fee included in interest expense was $56 for the nine months ended September 30, 2020 and 2019.
Income Tax Expense. Our effective tax rate was 21.0% and 19.7% for the three months ended September 30, 2020 and 2019, respectively. Our effective tax rate was 20.3% and 18.9% for the nine months ended September 30, 2020 and 2019. The effective tax rates for the three and nine months ended September 30, 2020 were lower than the statutory rate primarily due to the effects of tax-exempt investment income and the impact of stock-based compensation. The effective tax rates for the three and nine months ended September 30, 2019 were lower than the statutory rate primarily due to the effects of tax-exempt investment income.
Net Income. Net income for the three months ended September 30, 2020 was $44,742 compared to net income of $15,619 for the comparable 2019 period. Net income for the nine months ended September 30, 2020 was $85,246 compared to net income of $71,499 for the comparable 2019 period. The increase in both periods is a result of a decrease in loss and loss adjustment expenses as described above.
Non-GAAP Operating Income. Non-GAAP operating income as defined above was $38,128 for the three months ended September 30, 2020 compared to $15,079 for the comparable 2019 period. Non-GAAP operating income was $92,049 for the nine months ended September 30, 2020 compared to $58,635 for the comparable 2019 period.