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Assuming no coordination of benefits is needed between Swiss social security benefits and the US social security benefits, the Swiss social security benefits are taxed as social security benefits on the 1040, and as other income on the 540. (Technically, they should be taxed as a pension on the 540, but "other income" has the same tax consequences.)
If you qualify for Social Security benefits from both the United States and Switzerland and you did not need the agreement to qualify for either benefit, the amount of your U.S. benefit may be reduced. This is a result of a provision in U.S. law that can affect the way your benefit is figured if you also receive a pension based on work that was not covered by U.S. Social Security. For more information, call our toll-free number, 1-800-772-1213, and get the publication, Windfall Elimination Provision (Publication #05-10045).