critical estimates related to actuarial assumptions used to determine our pension expense and related obligation. We believe the most critical assumptions are related to (1) the discount rate used to determine the present value of the liabilities and (2) the expected long-term rate of return on plan assets. All of our actuarial assumptions are reviewed annually. Changes in these assumptions could have a material impact on the measurement of our pension expense and related obligation. 

At each measurement date, we determine the discount rate by reference to rates of high-quality, long-term corporate bonds that mature in a pattern similar to the future payments we anticipate making under the plans. As of December 31, 2014 and 2013, the weighted-average discount rate used to compute our benefit obligation was3.75 percent and 4.75 percent, respectively. 

The expected long-term rate of return on plan assets is based upon the long-term outlook of our investment strategy as well as our historical returns and volatilities for each asset class. We also review current levels of interest rates and inflation to assess the reasonableness of our long-term rates. Our pension plan investment objective is to ensure all of our plans have sufficient funds to meet their benefit obligations when they become due. As a result, the Company periodically revises asset allocations, where appropriate, to improve returns and manage risk. The weighted-average expected long-term rate of return used to calculate our pension expense was 8.25 percent in 2014 and 2013. 

Effective December 31, 2014, the Company revised our mortality assumptions used to determine the projected benefit obligation of the U.S. defined benefit pension plans. The revised assumptions were derived from the mortality tables and the mortality improvement scales published by the Society of Actuaries in October 2014. The change in mortality assumptions for the U.S. plans resulted in an increase in the projected benefit obligation at December 31, 2014, of approximately $210 million. 

In 2014, the Company's total pension expense related to defined benefit plans was$34 million. In 2015, we expect our total pension expense to be approximately $134 million. The anticipated increase is primarily due to a decrease in the weighted-average discount rate used to calculate the Company's benefit obligation, unfavorable asset performance compared to our expected return during 2014 and the adoption of more conservative mortality assumptions for U.S. plans offset by the impact of approximately $90 million of contributions the Company expects to make in 2015 to its international plans. The estimated impact of a 50 basis-point decrease in the discount rate on our 2015 pension expense is an increase to our pension expense of approximately $47 million. Additionally, the estimated impact of a 50 basis-point decrease in the expected long-term rate of return on plan assets on our 2015 pension expense is an increase to our pension expense of approximately $31 million. 

The sensitivity information provided above is based only on changes to the actuarial assumptions used for our U.S. pension plans. As ofDecember 31, 2014,