Selling, general, and administrative: Selling, general, and administrative (“SG&A”) expenses were $251.9 million for the three months ended September 30, 2021, an increase of $92.4 million, or 57.9%, as compared to $159.5 million from the corresponding period in 2020. As a percentage of revenue, SG&A expenses were 60.9% as compared to 58.7% for the three months ended September 30, 2021 and 2020, respectively. For the nine months ended September 30, 2021, SG&A expenses increased $277.5 million, or 69.0%, to $679.9 million from $402.4 million for the corresponding period in 2020. As a percentage of revenue, SG&A expenses were 59.2% for the nine months ended September 30, 2021 as compared to 60.1% for the corresponding period in 2020. The increase in SG&A for the quarter and nine months ended September 30, 2021 were primarily due to higher OPTAVIA commission expense, increased salaries and benefits related expenses for employees, incremental consulting costs related to information technology, increased credit card fees resulting from higher sales, as well as costs for the Company’s annual convention held in July 2021. As the OPTAVIA convention in July 2020 was a virtual event in response to the COVID-19 pandemic, the costs were significantly lower. SG&A expenses included research and development (“R&D”) costs of $1.2 million and $0.8 million for the three months ended September 30, 2021 and 2020, respectively, and $3.3 million and $1.9 million for the nine months ended September 30, 2021 and 2020, respectively.
OPTAVIA commission expense, which is a variable expense, increased $67.2 million, or 58.5%, to $182.1 million for the three months ended September 30, 2021 from $114.9 million for the corresponding period in 2020. For the nine months ended September 30, 2021, OPTAVIA commission expense increased $219.5 million, or 78.1%, to $500.7 million from $281.2 million for the corresponding period in 2020. The increase was primarily the result of increased OPTAVIA product sales. This trend is the result of the success we are experiencing with our growing OPTAVIA Integrated Coach Model. The total number of active earning OPTAVIA Coaches for the three months ended September 30, 2021 increased to 61,000 from 42,100 for the corresponding period in 2020, an increase of 44.9%. As OPTAVIA revenue increased as a portion of the Company’s total sales mix, the commission rate as a percentage of revenue increased 170 basis points to 44.0% for the third quarter of 2021 compared to 42.3% for the third quarter last year and increased 160 basis points to 43.6% for the nine months ended September 30, 2021 compared to 42.0% for the corresponding period in 2020.
Income from operations: For the three months ended September 30, 2021, income from operations increased $10.6 million to $55.2 million from $44.6 million for the corresponding period in 2020 primarily as a result of increased gross profit partially offset by increased SG&A expenses. Income from operations as a percentage of revenue was 13.3% and 16.4% for the three months ended September 30, 2021 and 2020, respectively. For the nine months ended September 30, 2021, income from operations increased $73.2 million to $169.4 million from $96.2 million for the corresponding period in 2020 primarily as a result of increased gross profits partially offset by increased SG&A expenses. Income from operations as a percentage of revenue was 14.8% and 14.4% for the nine months ended September 30, 2021 and 2020, respectively.
Provision for income tax: For the three months ended September 30, 2021, the Company recorded $13.2 million in income tax expense, an effective tax rate of 23.9%, as compared to $10.2 million in income tax expense, an effective tax rate of 22.8%, for the three months ended September 30, 2020. For the nine months ended September 30, 2021, the Company recorded $39.4 million in income tax expense, an effective rate of 23.2%, as compared to $21.6 million in income tax expense, an effective rate of 22.4%, for the nine months ended September 30, 2020. The increase in the effective tax rate for the quarter and nine months ended September 30, 2021 was primarily driven by an increase in the state income tax rate and limitations on the deductibility of officer compensation along with the tax benefit of stock compensation.
Net income: Net income was $42.0 million and $130.0 million, or $3.56 and $10.98 per diluted share, for the three and nine months ended September 30, 2021 as compared to $34.5 million and $74.9 million, or $2.91 and $6.32 per diluted share, for the three and nine months ended September 30, 2020. The period-over-period changes were driven by the factors described above in the explanations from operations.