"There are several reasons why Congress may choose to enact tax provisions on a temporary basis. Enacting provisions on a temporary basis provides legislators with an opportunity to evaluate the effectiveness of tax policies prior to expiration or extension. Temporary tax provisions may also be used to provide temporary economic stimulus or disaster relief. Congress may also choose to enact tax provisions on a temporary rather than permanent basis due to budgetary considerations, as the foregone revenue from a temporary provision will generally be less than if it was permanent. The provisions that expired at the end of 2013 are diverse in purpose, including provisions for individuals, businesses, the charitable sector, energy, community assistance, and disaster relief. Among the individual provisions that expired are deductions for teachers' out-of-pocket expenses, state and local sales taxes, qualified tuition and related expenses, and mortgage insurance premiums. On the business side, under current law, the R&D [Research and Development] tax credit, the WOTC [Work Opportunity Tax Credit], the active financing exceptions under Subpart F, and increased expensing and bonus depreciation allowances will not be available for taxpayers after 2013. Expired charitable provisions include the enhanced deduction for contributions of food inventory and provisions allowing for tax-free distributions from retirement accounts for charitable purposes. The renewable energy production tax credit (PTC) expired at the end of 2013, along with a number of other incentives for energy efficiency and renewable and alternative fuels. The new markets tax credit, a community assistance program, also expired at the end of 2013."
CRS Report for Congress, R43124