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 June 30, 2020 was $14.7 million, an increase of $0.2 million, or 1.7%, as compared to $14.5 million for the three months ended June 30, 2019. The Secured Note bears interest at 6% annually, and GCU makes monthly interest payments. The increase over the prior year was primarily due to an increase in the average principal balance of the Secured Note between periods due to net capital expenditure loans made to GCU under the Secured Note during the past twelve months.
Interest expense. Interest expense was $1.1 million for the three months ended June 30, 2020, a decrease of $1.8 million, as compared to interest expense of $2.9 million for the three months ended June 30, 2019. The decrease in interest expense was primarily due to a decline in the average credit facility outstanding balance between periods due to the paydown 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 June 30, 2020 was $0.4 million, a decrease of $2.3 million, as compared to $2.7 million in the three months ended June 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 June 30, 2020 was $15.3 million, an increase of $1.2 million, or 8.6%, as compared to income tax expense of $14.1 million for the three months ended June 30, 2019. This increase was the result of an increase in our effective tax rate between periods, partially offset by a decrease in taxable income. Our effective tax rate was 24.6% during the second quarter of 2020 compared to 21.7% during the second quarter of 2019. In the second quarter of 2020, the effective tax rate was impacted by higher state taxes and lower excess tax benefits of nil in the second quarter of 2020 as compared to $2.2 million in the same period in 2019 due to a lower stock price and lower stock option exercises in the second 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 June 30, 2020 was $47.0 million, a decrease of $4.1 million, or 8.0%, as compared to $51.1 million for the three months ended June 30, 2019, due to the factors discussed above.
Six Months Ended June 30, 2020 Compared to Six Months Ended June 30, 2019
Service revenue. Our service revenue for the six months ended June 30, 2020 was $407.4 million, an increase of $35.3 million, or 9.5%, as compared to service revenue of $372.1 million for the six months ended June 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% and an increase in revenue per student year over year. Partner enrollments in programs serviced by Orbis Education at June 30, 2020 were 3,720, an increase of 12.2% over 3,316 enrollments at June 30, 2019, while enrollments at GCU grew to 94,606, an increase of 8.0%. The increase in revenue per student is primarily due to the following factors (described below). The partnership agreements that were acquired as part of the Orbis Education acquisition generally generate a higher revenue per student than our agreement with GCU as these agreements generally have a higher percentage of service revenue, the partners have higher tuition rates than GCU and the majority of these students are studying in the Accelerated Bachelor of Science in Nursing program so these students take on average more credits per semester. Also, we generated slightly more Spring semester revenues in the first half of 2020 as compared to the first half of 2019 due to the timing of the Orbis Education acquisition on January 22, 2019, due to 2020 being a Leap Year and thus providing an extra day of revenue in 2020 as compared to 2019, and because our most significant university partner, GCU, had residential student enrollment growth year over year of 10.0% and residential students generate higher revenue per student than other GCU students due to ancillary revenues such as room and board. In addition, Orbis Education had five more locations opened in the second quarter of 2020 than it did in the prior year