six months ended June 30, 2021. The balance of the increase of $130.9 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 $3.3 million, or 9.0%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. The increase was due primarily to an increase in recordkeeping and administration fees and commissions.
Operating Expenses
Compensation and related expenses increased $48.3 million, or 20.9%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. The increase related to new partner firms was approximately $6.8 million. Non-cash equity compensation increased $8.3 million primarily associated with incentive unit grants and the modification of certain incentive units (see Note 9 to the unaudited condensed consolidated financial statements). The balance of the increase of $33.2 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 $57.6 million, or 35.8%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. The increase related to new partner firms was approximately $9.9 million. Management fees are variable and a function of earnings during the period. The balance of the increase of $47.7 million was primarily due to the increase in earnings during the six months ended June 30, 2021 compared to the six months ended June 30, 2020 and partner firm-level acquisitions.
Selling, general and administrative expenses increased $17.5 million, or 15.2%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. New partner firms added approximately $4.9 million. The balance of the increase of $12.6 million was due primarily to an increase in expenses related to information technology, rent and professional fees related to the growth of our existing partner firms and partner firm level acquisitions, and $1.4 million of costs related to the secondary offerings during the six months ended June 30, 2021.
Intangible amortization increased $15.3 million, or 21.3%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. The increase related to new partner firms was approximately $6.6 million. The balance of the increase of $8.7 million was due primarily to partner firm level acquisitions.
Non-cash changes in fair value of estimated contingent consideration increased $74.9 million for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. During the six months ended June 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 $1.2 million, or 20.0%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020 due primarily to office build-outs at certain partner firms.
Other income (expense)
Interest expense decreased $2.3 million, or 9.7%, for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. The decrease was due primarily to lower average interest rates on outstanding borrowings offset in part by higher average outstanding borrowings during the six months ended June 30, 2021 compared to the six months ended June 30, 2020.
During the six months ended June 30, 2020, a loss on extinguishment of borrowings of $6.1 million was recognized in connection with the January 2020 Credit Facility amendment.
Income Tax Expense
Income tax expense decreased $8.7 million for the six months ended June 30, 2021 compared to the six months ended June 30, 2020. For the six months ended June 30, 2021, we recorded tax expense based on an estimated annual effective tax rate of 27.9% and a discrete tax expense of approximately $0.3 million. The estimated annual effective tax