Research and Development
For the three months ended March 31, 2023, and 2022, research and development expense increased $0.3 million, or 52.9 percent to $0.8 million from $0.5 million for the same period last year. This is primarily due to higher consulting expenses related to next generation pump, payroll and benefits expenses offset by reductions in prototype expense.
Other Income, Net
Other income, net consists of interest income, foreign currency gains and losses, and other miscellaneous income. For the three months ended March 31, 2023, other income net increased $0.4 million, to $0.3 million from a loss of $(15,000) for the same period last year. This increase is due to interest received on money market fund investments.
Income Taxes
For the three months ended March 31, 2023, we recorded a provision for income tax expense of $943,589. Our effective tax rate was 21.7 percent and differed from the U.S. Federal statutory rate primarily due to U.S. state income tax expense, partially offset by benefits from foreign derived intangible income and research and development tax credits.
For the three months ended March 31, 2022, we recorded a provision for income tax expense of $573,295. Our effective tax rate was 18.7 percent and differed from the U.S. Federal statutory rate primarily due to benefits from foreign derived intangible income, stock compensation and research and development tax credits, partially offset by U.S. state income tax expense. For the full year 2023, we expect an effective tax rate of approximately 22 percent.
As of March 31, 2023, and December 31, 2022, we had not identified or accrued for any uncertain tax positions. We are currently unaware of any uncertain tax positions that could result in significant payments, accruals or other material deviations in this estimate over the next 12 months. We believe that our tax positions comply in all material respects with applicable tax law. However, tax law is subject to interpretation, and interpretations by taxing authorities could be different from ours, which could result in the imposition of additional taxes and penalties.
We file tax returns in the United States Federal jurisdiction and many U.S. state jurisdictions. Our returns are not currently under examination by the Internal Revenue Service. The Company remains subject to income tax examinations for our United States Federal and certain U.S. state income taxes for 2017 and subsequent years.
Liquidity and Capital Resources
Our principal sources of liquidity have historically been our cash and cash equivalents balances, our investments, cash flow from operations and access to the financial markets. Our principal uses of cash are operating expenses, working capital requirements, capital expenditures and dividend payments, if any.
As of March 31, 2023, we had cash and cash equivalents of $42.8 million, stockholders’ equity of $64.3 million, and working capital of $54.5 million. During the quarter ended March 31, 2023, we paid a special dividend of $13.2 million to our shareholders. As of December 31, 2022, we had cash and investments of $57.9 million, stockholders’ equity of $73.7 million, and working capital of $68.9 million.
We believe that our current cash, and any cash generated from operations will be sufficient to meet our ongoing operating requirements for at least the next 12 months. We have acquired land from an unrelated party and plan to make subsequent improvements thereon in the next two years to accommodate our increased operations and expand capacity. We anticipate using available cash and cash equivalents for that investment in our future growth. We do not anticipate