months ended March 31, 2026, was primarily attributable to a 4.5% decrease in material costs partially offset by a 2.1% increase in labor and assembly cost as a percentage of revenue as a result of as a result of customer and product mix.
Selling, General and Administrative Expenses
As a percentage of revenue, selling, general and administrative expenses decreased from 20.3% for the three months ended March 31, 2025, to 19.5% for the three months ended March 31, 2026. Selling, general and administrative expenses as a percentage of revenue will generally fluctuate whenever there is a significant fluctuation in revenue for the periods being compared.
Selling, general and administrative expenses increased 11.0% from $50.3 million for the three months ended March 31, 2025, to $55.8 million for the three months ended March 31, 2026. Selling, general and administrative expenses include personnel costs for management, accounting, information technology, human resources, sales and marketing, as well as professional fees, contract services and legal and litigation related costs. The increase in selling, general and administrative expenses was primarily attributable to increased employee-related costs of $3.7 million, increased deferred compensation plan expense of $1.5 million and increased travel related expenses of $0.6 million, partially offset by a decrease in stock-based compensation expense of $0.8 million. For the three months ended March 31, 2026, as compared to the three months ended March 31, 2025, changes in foreign currencies relative to the U.S dollar increased our selling, general and administrative expenses by approximately $2.3 million.
Research and Development Expenses
As a percentage of revenue, research and development expenses decreased from 19.7% for the three months ended March 31, 2025, to 17.7% for the three months ended March 31, 2026. Research and development expenses as a percentage of revenue will generally fluctuate whenever there are incremental product development activities or significant fluctuations in revenue for the periods being compared.
Research and development expenses increased 3.9% from $48.9 million for the three months ended March 31, 2025, to $50.8 million for the three months ended March 31, 2026. The increase in research and development expenses for the three months ended March 31, 2026, was primarily attributable to increased employee-related costs of $2.6 million, increased office lease expense of $0.4 million and increased depreciation and amortization expense of $0.4 million partially offset by increased governmental research and development subsidies of $0.8 million and decreased stock-based compensation expense of $0.4 million. For the three months ended March 31, 2026, as compared to the three months ended March 31, 2025, changes in foreign currencies relative to the U.S. dollar increased our research and development expenses by approximately $2.3 million.
Adtran Networks has arrangements with governmental entities for the purpose of obtaining funding for research and development activities. The Company classifies government grants received under these arrangements as a reduction to research and development expenses incurred. For the three months ended March 31, 2026 and 2025, the Company recognized $3.1 million and $2.2 million as a reduction of research and development expense, respectively.
We expect to continue to incur research and development expenses in connection with our new and existing products. We continually evaluate new product opportunities and engage in significant research and product development efforts, which provides for new product development, enhancement of existing products and product cost reductions. We may incur significant research and development expenses prior to the receipt of revenue from a major new product group.
Interest and Dividend Income
Interest and dividend income increased from $0.1 million for the three months ended March 31, 2025 to $0.3 million for the three months ended March 31, 2026. The increase in interest and dividend income is primarily attributable to fluctuations in investment balances and an increase in the rate of return on those investments due to interest rate movements.
Interest Expense
Interest expense decreased from $4.8 million for the three months ended March 31, 2025, to $4.2 million for the three months ended March 31, 2026. The decrease in interest expense during the three months ended March 31, 2026, was primarily driven by the issuance of the 2030 Notes, which accrues interest at 4.7%, and the repayment of the majority of the Credit Agreement which accrued interest at 8.6% for the three months ended March 31, 2025. See Note 10 and Note 11 of the Notes to Condensed Consolidated Financial Statements, included in Part I, Item 1 of this report for additional information.
Net Investment Loss
We recognized a net investment loss of $1.7 million for the three months ended March 31, 2025 and a net investment loss of $0.9 million for the three months ended March 31, 2026. The fluctuations in our net investments were primarily attributable to changes in the fair value of our securities recognized during the period. We expect that any future market volatility could result in continued fluctuations in our investment portfolio. See Note 5 of the Notes to Condensed Consolidated Financial Statements, included in Part I, Item 1 of this report, and “Investing Activities” in “Liquidity and Capital Resources” below for additional information.