Huh? In the US the married filing jointly tax brackets are exactly double the single tax brackets for every rate except the top 37% rate. A single person making 100k definitely pays a lot more in tax than than a married couple making 100k together. It's generally advantageous to be married filing jointly unless you're at the absolute top 37% rate, at the very bottom (where means tested benefits phase out), or both spouses make roughly equal incomes (in which case MFJ vs two single filers works out around the same).