Revenues
Wealth management fees increased $122.2 million, or 39.2%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. New partner firms added subsequent to the three months ended September 30, 2020 that are included in our results of operations for the three months ended September 30, 2021 include InterOcean Capital, Seasons of Advice, CornerStone Partners, Fairway Wealth Management, Kavar Capital Partners, Hill Investment Group, Prairie Capital Management, Rollins Financial, ARS Wealth Advisors and Badgley Phelps Wealth Managers. Additionally, our partner firms completed 21 acquisitions subsequent to the three months ended September 30, 2020. The new partner firms contributed approximately $27.6 million in revenue during the three months ended September 30, 2021. The balance of the increase of $94.6 million was due to the revenue growth at our existing partner firms associated with wealth management services, which includes partner firm-level acquisitions.
Other revenues increased $0.9 million, or 4.5%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. Other revenues from new partner firms was approximately $0.1 million. The balance of the increase of $0.8 million was due primarily to an increase in recordkeeping and administration fees.
Operating Expenses
Compensation and related expenses increased $29.5 million, or 25.8%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. The increase related to new partner firms was approximately $5.4 million. Non-cash equity compensation increased $0.6 million primarily associated with incentive unit and stock option grants. The balance of the increase of $23.5 million was due primarily to an increase in salaries and related expense due to the growth of existing partner firms and partner firm-level acquisitions.
Management fees increased $40.8 million, or 47.2%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. The increase related to new partner firms was approximately $7.3 million. Management fees are variable and a function of earnings during the period. The balance of the increase of $33.5 million was primarily due to the increase in earnings during the three months ended September 30, 2021 compared to the three months ended September 30, 2020 and partner firm-level acquisitions.
Selling, general and administrative expenses increased $18.2 million, or 31.7%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. New partner firms added approximately $4.3 million. The balance of the increase of $13.9 million was due primarily to an increase in expenses related to professional fees, information technology, travel and entertainment, and marketing and business development related to the growth of our existing partner firms and partner firm-level acquisitions.
Intangible amortization increased $9.0 million, or 24.4%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. The increase related to new partner firms was approximately $5.2 million. The balance of the increase of $3.8 million was due primarily to partner firm-level acquisitions.
Non-cash changes in fair value of estimated contingent consideration increased $22.0 million, or 153.8%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. During the three months ended September 30, 2021, the probability that certain contingent consideration payments would be achieved increased due to Monte Carlo Simulation changes associated with market conditions and forecasts, resulting in an increase in the fair value of the contingent consideration liability.
Depreciation and other amortization expense increased $0.5 million, or 16.1%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020 due primarily to office build-outs at certain partner firms.
Other income (expense)
Interest expense increased $7.6 million, or 85.8%, for the three months ended September 30, 2021 compared to the three months ended September 30, 2020. The increase was due primarily to higher average outstanding borrowings during the three months ended September 30, 2021 compared to the three months ended September 30, 2020.