development efforts are a key element of our strategy and are essential to our success, and we intend to maintain our commitment to research and development. However, increase or decrease in our revenue would not necessarily result in a proportional increase or decrease in the levels of our research and development expenditures, which could affect our operating margin.
Selling, General and Administrative. Selling, general and administrative expense, which is primarily comprised of compensation expense, increased by $6.2 million, or 5.1%, to $128.1 million in the three months ended December 31, 2021, from $121.9 million in the three months ended December 31, 2020. Selling, general and administrative expense increased as a percentage of revenue from 11.2% in the three months ended December 31, 2020, to 11.6% in the three months ended December 31, 2021. The increase was primarily attributable to sales and marketing efforts, which were partially offset by decrease in the account receivable allowances. Selling, general and administrative expense may fluctuate from time to time, depending upon such factors as changes in our workforce and sales efforts and the results of any operational efficiency programs that we may undertake.
Amortization of Purchased Intangible Assets and Other. Amortization of purchased intangible assets and other in the three months ended December 31, 2021, decreased by $2.1 million, or 10.7% to $17.7 million from $19.9 million in the three months ended December 31, 2020. The decrease in amortization of purchased intangible assets and other was primarily attributable to a completion of amortization of previously purchased intangible assets, partially offset by an increase in amortization of intangible assets due to recently completed acquisitions.
Operating Income. Operating income increased by $19.9 million, or 14.2%, in the three months ended December 31, 2021, to $160.1 million, or 14.5% of revenue, from $140.2 million, or 12.9% of revenue, in the three months ended December 31, 2020. The increase in operating income was attributable primarily to the increase in operational excellence initiatives, the divestiture of OpenMarket completed on December 31, 2020 as well as to changes in certain acquisition-related liabilities measured at fair value, partially offset by increase in research and development expense and increase in selling, general and administrative expense. Our operating income was negatively affected by foreign exchange impacts.
Interest and Other Expense, Net. Interest and other expense, net, changed from a net expense of $6.5 million in the three months ended December 31, 2020 to a net expense of $2.6 million in the three months ended December 31, 2021. The decrease in interest and other expense, net, was primarily attributable to changes of minority equity investments measured at fair value in the three months ended in December 2021, partially offset by foreign exchange impacts.
Gain from sale of a business. Gain from sale of a business, in the three months ended December 31, 2021, decreased by $216.4 million, or 95.6% to $10.0 million from $226.4 million in the three months ended December 31, 2020. Please see Note 3 to our consolidated financial statements.
Income Taxes. Income taxes for the three months ended December 31, 2021 were $34.0 million on pre-tax income of $167.6 million, resulting in an effective tax rate of 20.3%, compared to 16.8% in the three months ended December 31, 2020. Our effective tax rate may fluctuate between periods as a result of discrete items that may affect a particular period. Please see Note 9 to our consolidated financial statements.
Net Income. Net income decreased by $166.0 million, or 55.4%, to $133.6 million in the three months ended December 31, 2021, from $299.6 million in the three months ended December 31, 2020. The decrease in net income was primarily attributable to the gain from sale of a business, net of tax, which was recorded in the three months ended December 31, 2020.
Diluted Earnings Per Share. Diluted earnings per share decreased by $1.21, or 53.1%, to $1.07 in the three months ended December 31, 2021, from $2.28 in the three months ended December 31, 2020. The decrease in diluted earnings per share was primarily attributable to the gain from sale of a business, net of tax, which increased the diluted earnings per share by $0.06 and $1.42 for the three months ended December 31, 2021 and 2020, respectively. The decrease was partially offset by the decrease in the diluted weighted average number of shares outstanding, which resulted from share repurchases. Please see also Note 10 to our consolidated financial statements.
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