assets (real estate, energy, and timber). The actual allocations to these asset classes may vary tactically around the long-term policy allocations based on relative market valuations. Our historical investment returns (compound annual growth rates) for our United States defined benefit pension and other postretirement plan assets were a 17 percent gain in the 1 year period ended May 30, 2010, and returns of 5 percent, 6 percent, 9 percent, and 10 percent for the 5, 10, 15, and 20 year periods ended May 30, 2010. Our principal defined benefit pension and other postretirement plans in the United States have an expected return on plan assets of 9.6 percent. On a weighted-average basis, the expected rate of return for all defined benefit plans was 9.55 percent for fiscal 2010, 9.55 percent for fiscal 2009, and 9.56 percent for fiscal 2008. Lowering the expected long-term rate of return on assets by 50 basis points would increase our net pension and postretire- ment expense by $23 million for fiscal 2011. A market-related valuation basis is used to reduce year-to-year expense volatility. The market-related valuation recognizes certain investment gains or losses over a five-year period from the year in which they occur. Investment gains or losses for this purpose are the differ- ence between the expected return calculated using the market- related value of assets and the actual return based on the market- related value of assets. Our outside actuaries perform these calculations as part of our determination of annual expense or income. Discount Rates Our discount rate assumptions are determined annually as of the last day of our fiscal year for all of our defined benefit pension, other postretirement, and postemployment ben- efit plan obligations. We also use the same discount rates to determine defined benefit pension, other postretirement, and postemployment benefit plan income and expense for the fol- lowing fiscal year. We work with our actuaries to determine the timing and amount of expected future cash outflows to plan participants and, using the top quartile of AA-rated corporate bond yields, to develop a forward interest rate curve, including a margin to that index based on our credit risk.This forward interest rate curve is applied to our expected future cash outflows to determine our discount rate assumptions. Our weighted-average discount rates were as follows: Weighted-Average Discount Rates Lowering the discount rates by 50 basis points would increase our net defined benefit pension, other postretirement, and post- employment benefit plan expense for fiscal 2011 by approximately $31 million. All obligation-related experience gains and losses are amortized using a straight-line method over the average remain- ing service period of active plan participants. Health Care Cost Trend Rates We review our health care cost trend rates annually. Our review is based on data we collect about our health care claims experience and information provided by our actuaries. This information includes recent plan experience, plan design, overall industry experience and projections, and assumptions used by other similar organizations. Our initial health care cost trend rate is adjusted as necessary to remain consistent with this review, recent experiences, and short-term expectations. Our initial health care cost trend rate assumption is 9.0 percent for all retirees. Rates are graded down annually until the ultimate trend rate of 5.2 percent is reached in 2019 for all retirees. The trend rates are applicable for calculations only if the retirees’ benefits increase as a result of health care inflation. The ultimate trend rate is adjusted annually, as necessary, to approx- imate the current economic view on the rate of long-term infla- tion plus an appropriate health care cost premium. Assumed trend rates for health care costs have an important effect on the amounts reported for the other postretirement benefit plans. Defined Benefit Pension Plans Other Postretirement Benefit Plans Postemployment Benefit Plans ......................................................................................................................................................................................... Obligations as of May 30, 2010, and fiscal 2011 expense 5.85% 5.80% 5.12% Obligations as of May 31, 2009, and fiscal 2010 expense 7.49% 7.45% 7.06% Fiscal 2009 expense 6.88% 6.90% 6.64% General Mills 40