From the Document: "Social Security spousal benefits were established in the 1930s to help support wives who are financially dependent on their husbands. It has since become more common for both spouses in a couple to work, with the result that, in more cases, both members of a couple are entitled to Social Security or other government pensions based on their own work records. Social Security does not provide both a full retired-worker and a full spousal benefit to the same individual. Two provisions are designed to reduce the Social Security spousal benefits of individuals who are not financially dependent on their spouses because they receive benefits based on their own work records. These are  the 'dual entitlement' rule, which applies to spouses who qualify for both (1) Social Security spousal benefits based on their spouses' work histories in Social Security-covered employment and (2) their own Social Security retired- or disabled-worker benefits, based on their own work histories in Social Security-covered employment; and  the Government Pension Offset (GPO), which applies to spouses who qualify for both (1) Social Security spousal benefits based on their spouses' work histories in Social Security-covered employment and (2) their own government pensions, based on their own work in government employment that was not covered by Social Security. The GPO reduces Social Security spousal or widow(er)'s benefits by two-thirds of the pension from non-covered government employment. The GPO does not reduce the benefits of the spouse who was covered by Social Security."
CRS Report for Congress, RL32453