relating to the income previously noted. While there may be opportunity for our U.S. operations to generate profits in the  future, our near-term level of profitability is uncertain.  The potential long-term profitability cannot be given as much weight in  our analysis given the objectively verifiable lack of sustained pro forma historical profitability and uncertainty associated with  the future U.S. operations. To the extent that our operations in the U.S., after giving ef fect to the planned aforementioned tax  actions, are profitable in 2015 and our projections of profitability beyond 2015 are suf ficiently positive based upon objective  and verifiable assumptions, it is reasonably possible that we could release up to $500 of the remaining valuation allowance  against our U.S. deferred tax assets in the next twelve months. In the ordinary course of business, there are many transactions and calculations where the ultimate tax determination is less  than certain. We are regularly under audit by the various applicable tax authorities.  Although the outcome of tax audits is  always uncertain, we believe that we have appropriate support for the positions taken on our tax returns and that our annual tax  provisions include amounts sufficient to pay assessments, if any , which may be proposed by the taxing authorities. Nonetheless,  the amounts ultimately paid, if any , upon resolution of the issues raised by the taxing authorities may dif fer materially from the  amounts accrued for each year . See additional discussion of our deferred tax assets and liabilities in Note 17 to our consolidated  financial statements in Item 8. Retiree benefits  — Accounting for pensions and other postretirement benefits (OPEB) involves estimating the cost of benefits  to be provided well into the future and attributing that cost to the time period each employee works.  These plan expenses and  obligations are dependent on assumptions developed by us in consultation with our outside advisers such as actuaries and other  consultants and are generally calculated independently of funding requirements.  The assumptions used, including inflation,  discount rates, investment returns, life expectancies, turnover rates, retirement rates, future compensation levels and health care  cost trend rates, have a significant impact on plan expenses and obligations.  These assumptions are regularly reviewed and  modified when appropriate based on historical experience, current trends and the future outlook. Changes in one or more of the  underlying assumptions could result in a material impact to our consolidated financial statements in any given period. If actual  experience differs from expectations, our financial position and results of operations in future periods could be af fected. Mortality rates are based in part on the company's plan experience and actuarial estimates.The inflation assumption is  based on an evaluation of external market indicators, while retirement and turnover rates are based primarily on actual plan  experience. Health care cost trend rates are developed based on our actual historical claims experience, the near -term outlook  and an assessment of likely long-term trends. For our lar gest plans, discount rates are based upon the construction of a  theoretical bond portfolio, adjusted according to the timing of expected cash flows for the future obligations.  A yield curve is  developed based on a subset of these high-quality fixed-income investments (those with yields between the 40th and 90th  percentiles). The projected cash flows are matched to this yield curve and a present value developed which is then calibrated to  develop a single equivalent discount rate. Pension benefits are funded through deposits with trustees that satisfy , at a minimum,  the applicable funding regulations. For our lar gest defined benefit pension plans, expected investment rates of return are based  on input from the plans’ investment advisers and actuary regarding our expected investment portfolio mix, historical rates of  return on those assets, projected future asset class returns, the impact of active management and long-term market conditions  and inflation expectations. We believe that the long-term asset allocation on average will approximate the tar geted allocation  and we regularly review the actual asset allocation to periodically re-balance the investments to the tar geted allocation when  appropriate. OPEB benefits are funded as they become due. Actuarial gains or losses may result from changes in assumptions or when actual experience is dif ferent from that which  was expected. Under the applicable standards, those gains and losses are not required to be immediately recognized in our  results of operations as income or expense, but instead may be deferred as part of accumulated other comprehensive income  (AOCI) and amortized into our results of operations over future periods. Our U.S. defined benefit pension plans comprise about 85% of our consolidated defined benefit pension obligations at December 31, 2014. These plans are frozen and no service-related costs are being incurred. Changes in our net obligations are  principally attributable to changing discount rates and the performance of plan assets. Pension obligations are valued using  discount rates established annually in consultation with our outside actuarial advisers using a theoretical bond portfolio,  adjusted according to the timing of expected cash flows for our future obligations. Declining discount rates increase the  present value of future pension obligations – a 25 basis point decrease in the discount rate would increase our U.S. pension  liability by about $47. As indicated above, when establishing the expected long-term rate of return on our U.S. pension plan  assets, we consider historical performance and forward looking return estimates reflective of our portfolio mix and expected  investment strategy. Our investment strategy and portfolio complexion is described in Note 1 1 of the consolidated financial  statements in Item 8. Although actual annual returns over the past five years have averaged more than 10%, based on the  U.S. retirement plans 34