Did you claim the foreign earned income exclusion? If so, you have to use the Foreign Earned Income Tax Worksheet to calculate your tax. You can't just calculate it using the taxable income on Form 1040 line 15. The amount of excluded foreign income affects the tax because your other income, that is not excluded, is taxed at the same rates that would have applied to that income if you did not get the exclusion. The non-excluded income is "stacked" on top of the excluded income for the purpose of calculating the tax. Therefore, when the amount of excluded foreign income increases, some of the other income gets pushed into a higher tax bracket, and is taxed at a higher percentage. The Foreign Earned Income Tax Worksheet does all the necessary calculations.
If there is an amount on Schedule 1 line 8d you are claiming the foreign earned income exclusion.
