operating expenses to deliver those services, partially offset by our ability to leverage our other general and administrative expenses across an increasing revenue base.
Amortization of intangible assets. Amortization of intangible assets for the three months ended September 30, 2020 was $2.1 million a decrease of $0.1 million as compared to amortization of intangible assets for the three months ended September 30, 2019 of $2.2 million. As a result of the Acquisition, certain identifiable intangible assets were created (primarily customer relationships) that will be amortized over their expected lives.
Interest income on Secured Note. Interest income on the secured note from GCU in the initial principal amount of $870.1 million (the “Secured Note”) for the three months ended September 30, 2020 was $14.9 million, a decrease of $1.3 million, or 8.2%, as compared to $16.2 million for the three months ended September 30, 2019. The Secured Note bears interest at 6% annually, and GCU makes monthly interest payments. The decrease over the prior year was primarily due to a decrease in the average principal balance of the Secured Note between periods due to net loan repayments made by GCU under the Secured Note during the past twelve months. In September 2019, GCU repaid $60.0 million and in October 2019 GCU repaid $40.0 million. The $75.0 million borrowed by GCU at the end of June 2020 was repaid in early July 2020.
Interest expense. Interest expense was $0.9 million for the three months ended September 30, 2020, a decrease of $2.0 million, as compared to interest expense of $2.9 million for the three months ended September 30, 2019. The decrease in interest expense was primarily due to a decline in the average credit facility outstanding balance between periods due to paydowns of the credit facility during the past twelve months and an interest rate reduction of approximately 150 basis points since the end of 2019.
Investment interest and other. Investment interest and other for the three months ended September 30, 2020 was $0.2 million, a decrease of $0.1 million, as compared to $0.3 million in the three months ended September 30, 2019. This decrease was primarily attributable to a decline in interest income on excess cash as the average investment balance declined year over year and lower interest rates.
Income tax expense. Income tax expense for the three months ended September 30, 2020 was $13.1 million, a decrease of $2.1 million, or 13.4%, as compared to income tax expense of $15.2 million for the three months ended September 30, 2019. This decrease was the result of lower taxable income and a slight decrease in our effective tax rate between periods. Our effective tax rate was 20.2% during the third quarter of 2020 compared to 20.7% during the third quarter of 2019. In the third quarter of 2020, the effective tax rate was impacted by a $1.0 million increase in contributions made in lieu of state income taxes to school sponsoring organizations from $4.0 million for the three months ended September 30, 2019 to $5.0 million for the three months ended September 30, 2020, partially offset by lower excess tax benefits, which declined to $0.1 million in the third quarter of 2020 as compared to $0.4 million in the same period in 2019 due to a lower stock price and fewer stock option exercises in the third quarter of 2020. The inclusion of excess tax benefits and deficiencies as a component of our income tax expense will increase volatility within our provision for income taxes as the amount of excess tax benefits or deficiencies from share-based compensation awards are dependent on our stock price at the date the restricted awards vest, our stock price on the date an option is exercised, and the quantity of options exercised. Our restricted stock vests in March each year so the favorable benefit will primarily impact the first quarter each year.
Net income. Our net income for the three months ended September 30, 2020 was $52.0 million, a decrease of $6.2 million, or 10.5%, as compared to $58.2 million for the three months ended September 30, 2019, due to the factors discussed above.
Nine Months Ended September 30, 2020 Compared to Nine Months Ended September 30, 2019
Service revenue. Our service revenue for the nine months ended September 30, 2020 was $605.8 million, an increase of $40.4 million, or 7.1%, as compared to service revenue of $565.4 million for the nine months ended September 30, 2019. The increase year over year in service revenue was primarily due to an increase in university partner enrollments between years of 8.2% partially offset by a decrease in revenue per student year over year. Partner enrollments totaled 117,772 at September 30, 2020 as compared to 108,821 at September 30, 2019. Enrollments at GCU