Differences in mortality play an important role in redistributions. SS requires that benefits be annuitized and that all payments into the system immediately go toward the purchase of the annuity. This helps guarantee against poverty in old age while helping to maintain the goal of horizontal equity or equal treatment of equals.

Women live longer than men, and since the same annuity rate is applied regardless of sex, this factor by itself would redistribute lifetime benefits relative to taxes across the sexes. In addition, some lower-income individuals and minority groups with lower-than-average incomes experience higher mortality rates.