the three months ended June 30, 2023, from 24.9% for the three months ended June 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 June 30, 2023 were $10.9 million, an increase of $1.0 million, or 10.3%, as compared to general and administrative expenses of $9.9 million for the three months ended June 30, 2022. This increase was primarily attributable to increased charitable contributions and increased professional fees of $0.7 million and $0.5 million, respectively, partially offset by a decrease in employee compensation, including share-based expenses of $0.2 million. Our general and administrative expenses as a percentage of revenue increased by 0.3% to 5.2% for the three months ended June 30, 2023, from 4.9% for the three months ended June 30, 2022.
Amortization of intangible assets. Amortization of intangible assets for the three months ended June 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 June 30, 2023 was $2.6 million, as compared to investment interest and other for the three months ended June 30, 2022 of $0.3 million due to higher investment balances and higher returns on those balances.
Income tax expense. Income tax expense for the three months ended June 30, 2023 was $9.1 million, an increase of $0.5 million, or 5.0%, as compared to income tax expense of $8.6 million for the three months ended June 30, 2022. This increase was the result of an increase in our taxable income, partially offset by a decrease in our effective tax rate between periods. Our effective tax rate was 23.8% during the second quarter of 2023 compared to 25.2% during the second quarter of 2022. In the second quarter of 2023 the effective tax rate was favorably impacted by state tax audits, while in the second quarter of 2022 the effective tax rate was unfavorably impacted by an increase in the state income tax rate.
Net income. Our net income for the three months ended June 30, 2023 was $29.0 million, an increase of $3.4 million, or 13.3%, as compared to $25.6 million for the three months ended June 30, 2022, due to the factors discussed above.
Six Months Ended June 30, 2023 Compared to Six Months Ended June 30, 2022
Service revenue. Our service revenue for the six months ended June 30, 2023 was $460.7 million, an increase of $16.8 million, or 3.8%, as compared to service revenue of $443.9 million for the six months ended June 30, 2022. The increase year over year in service revenue was primarily due to an increase in GCU enrollments to 95,972 at June 30, 2023, an increase of 4.1% over enrollments at June 30, 2022 and an increase in revenue per student year over year. The increase in revenue per student between years is primarily due to the service revenue impact of the increased room, board and other ancillary revenues at GCU’s ground traditional campus between years primarily due to increased enrollment. In addition, service revenue per student for ABSN students at off-campus classroom and laboratory sites generates a significantly higher revenue per student than we earn under our agreement with GCU, as these agreements generally provide us with a higher revenue share percentage, the partners have higher tuition rates than GCU and the majority of their students take more credits on average per semester. Partner enrollments totaled 99,526 at June 30, 2023 as compared to 96,029 at June 30, 2022. University partner enrollments at our off-campus classroom and laboratory sites were 3,904, a decrease of 5.2% over enrollments at June 30, 2022, which includes 350 and 324 GCU students at June 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