the three months ended September 30, 2023, from 24.3% for the three months ended September 30, 2022, primarily due to our ability to leverage our marketing and communication expenses across an increasing revenue base, partially offset by the number of university partners and their growth expectations and increased off-campus classroom and laboratory sites opened and sites planned to open in the next 12 months.
General and administrative. Our general and administrative expenses for the three months ended September 30, 2023 were $12.2 million, a decrease of $3.4 million, or 21.8%, as compared to general and administrative expenses of $15.6 million for the three months ended September 30, 2022. This decrease was primarily attributable to a decrease in the contribution made in lieu of state income taxes, decreased professional fees, decreased other administrative expenses, and decreased employee compensation, including share-based expenses and benefits of $1.5 million, $1.2 million, $0.5 million and $0.2 million respectively. We decreased our contribution made in lieu of state income taxes from $5.0 million in 2022 to $3.5 million in 2023. Our professional fees declined between years primarily due to lower legal costs as we met our insurance retention cap on a litigation matter. Our general and administrative expenses as a percentage of revenue decreased by 2.0% to 5.5% for the three months ended September 30, 2023, from 7.5% for the three months ended September 30, 2022.
Amortization of intangible assets. Amortization of intangible assets for the three months ended September 30, 2023 and 2022 were $2.1 million for both periods. As a result of the acquisition of our wholly owned subsidiary, Orbis Education, certain identifiable intangible assets were created (primarily customer relationships) that will be amortized over their expected lives.
Investment interest and other. Investment interest and other for the three months ended September 30, 2023 was $2.7 million, as compared to investment interest and other for the three months ended September 30, 2022 of $0.7 million due to higher investment balances and higher returns on those balances.
Income tax expense. Income tax expense for the three months ended September 30, 2023 was $8.5 million, an increase of $2.3 million, or 36.6%, as compared to income tax expense of $6.2 million for the three months ended September 30, 2022. Our effective tax rate was 19.3% during the third quarter of 2023 compared to 17.2% during the third quarter of 2022. The increase in our effective tax rate between periods was primarily driven by changes in the magnitude of contributions in lieu of state income taxes as compared to prior periods.
Net income. Our net income for the three months ended September 30, 2023 was $35.7 million, an increase of $5.7 million, or 19.1%, as compared to $30.0 million for the three months ended September 30, 2022, due to the factors discussed above.
Nine Months Ended September 30, 2023 Compared to Nine Months Ended September 30, 2022
Service revenue. Our service revenue for the nine months ended September 30, 2023 was $682.6 million, an increase of $30.0 million, or 4.6%, as compared to service revenue of $652.6 million for the nine months ended September 30, 2022. The increase year over year in service revenue was primarily due to an increase in GCU enrollments to 118,227 at September 30, 2023, an increase of 6.6% over enrollments at September 30, 2022. Partner enrollments totaled 123,165 at September 30, 2023 as compared to 116,202 at September 30, 2022. University partner enrollments at our off-campus classroom and laboratory sites were 5,448, a decrease of 4.3% over enrollments at September 30, 2022, which includes 510 and 421 GCU students at September 30, 2023 and 2022, respectively. None of our ABSN partners stopped admitting new students due to the clinical faculty challenges that began during the pandemic, however some locations that were scheduled to open were delayed and some existing partners have experienced reduced incoming cohort sizes which has slowed the growth. We believe the growth in the number of ABSN students is also being negatively impacted by the strong job market as these students have historically been individuals with already completed bachelor’s degrees choosing to re-career into one of these health professions. To address this challenge, we have been working with a number of our university partners to adjust their programs to allow students with the required education experience but without a complete bachelor’s degree to enter their programs. We did open six new off-campus classroom and laboratory sites in the year ended December 31, 2022 and five sites in the nine months ended September 30, 2023 increasing the total number of these sites to 40 at September 30, 2023. Enrollments for GCU ground students were 25,232 at September 30, 2023 up from 25,083 at September 30, 2022. GCU