Aon net income attributable to Aon shareholders in the fourth quarter increased 64% to $863 million, or $3.90 per share, compared to $525 million, or $2.27 per share, in the prior year period.
Net income per share, adjusted for certain items, increased 42% to $3.71, including an unfavorable impact of $0.03 per share if prior year period results were translated at current period foreign exchange rates ("foreign currency translation"), compared to $2.62 in the prior year period.
Certain items that impacted fourth quarter results and comparisons with the prior year period are detailed in "Reconciliation of Non-GAAP Measures - Operating Income and Diluted Earnings Per Share" on page 10 of this press release.
Total revenue in the fourth quarter increased 4% to $3.1 billion compared to the prior year period driven by 10% organic revenue growth, partially offset by a 5% unfavorable impact from divestitures, net of acquisitions, and a 1% unfavorable impact from foreign currency translation.
Total operating expenses in the fourth quarter decreased 6% to $2.1 billion compared to the prior year period due primarily to a $200 million positive impact from the repatterning of discretionary expenses within the year, as described in previous periods, a $64 million decrease in expenses related to divestitures, net of acquisitions, a $44 million decrease in transaction costs, and a $12 million favorable impact from foreign currency translation, partially offset by an increase in expense associated with 10% organic revenue growth and investments in long-term growth.
Foreign currency translation in the fourth quarter had a $6 million, or $0.03 per share, unfavorable impact on U.S. GAAP net income and a $7 million, or $0.03 per share, unfavorable impact on adjusted net income. If currency were to remain stable at today's rates, the Company would expect an unfavorable impact of approximately $0.16 per share, or an approximately $48 million decrease in operating income, in the first quarter of 2022.
Effective tax rate used in the Company's U.S. GAAP financial statements for the fourth quarter was 15.7%, compared to 14.7% in the prior year period, primarily driven by changes in the geographical distribution of income and a net favorable impact from discrete items.
After adjusting to exclude the applicable tax impact associated with certain non-GAAP adjustments, the adjusted effective tax rate for the fourth quarter decreased to 12.2% compared to 13.4% in the prior year period, also primarily driven by changes in the geographical distribution of income and a net favorable impact from discrete items.
The prior year period also included a net favorable impact from discrete items.
These adjustments are discussed in "Reconciliation of Non-GAAP Measures - Operating Income and Diluted Earnings Per Share" on page 10 of this press release.
Weighted average diluted shares outstanding decreased to 221.2 million in the fourth quarter compared to 230.9 million in the prior year period.
The Company repurchased 6.7 million Class A Ordinary Shares for approximately $2.0 billion in the fourth quarter. As of December 31, 2021, the Company had approximately $1.7 billion of remaining authorization under its share repurchase program. ■