The objective of this study is to investigate the magnitude of the increase in differential mortality and its impact on the progressivity of the retirement system. Several studies have incorporated differential mortality in constructed measures of lifetime contributions and benefits; and they frequently conclude that mortality differences are sufficient to offset large portions of the progressivity that was originally built into the Social Security system. However, most of those studies were limited to the retirement portion of the OASDI program. Less account has been taken of disability and survivor benefits. As highlighted by a recent CBO (2006) report, most of the lifetime progressivity flows from the disability and survivor portions of the program.