Marriage changes your tax bill before it changes anything else. Most thresholds in the code double for couples filing jointly — the tax brackets and the standard deduction (Table 6.1) among them — so for many couples filing jointly is a wash or a modest gain. The marriage bonus appears when a high earner marries a low or non-earner: the second spouse’s standard deduction and lower brackets absorb part of the first spouse’s income. Income that would have been taxed at 32–35% as a single filer drops into the 22–24% range. A stay-at-home spouse is, among other things, a tax shelter.
The marriage penalty appears when two high earners marry, because a handful of thresholds were never doubled. Consider two professionals, each with a $500,000 salary. As single filers, each has taxable income near $483,900 after the 2026 standard deduction; the top 37% bracket begins at $626,350, so neither reaches it — their last dollars are taxed at 35%. Marry them and file jointly: combined taxable income is roughly $967,800, and the 37% bracket for joint filers begins at only $751,600 — not the $1,252,700 a true doubling would produce. About $216,000 of their income is now taxed at 37% instead of 35%, a two-point surcharge worth roughly $4,300, owed for nothing more than signing a marriage certificate. The same non-doubling appears elsewhere: the SALT deduction cap is the identical dollar figure for a single filer and a couple, and the thresholds for the section “Net Investment Income Tax (NIIT)” are far from doubled. Two stacked high incomes collide with all of them at once.
Filing separately rarely cures the penalty — the married-filing-separately brackets are punitive and the status disables many credits (section “Filing status”). Reserve it for specific situations: shrinking a spouse’s income-driven student-loan payment, isolating yourself from liability for a spouse’s questionable return, or clearing an AGI-gated deduction threshold on one spouse’s lower income. The harshest version of the penalty is not on this list at all — it falls on a surviving spouse, whose brackets compress from joint to single overnight (section “The Widow’s Penalty”).