A more thorough explanation:

Definition: Marriage penalty refers to the additional income tax liability that a married couple has to pay when filing a joint income tax return, compared to what they would have paid if they were single and filed individual returns. This happens when the tax code treats married couples unfavorably in comparison to unmarried couples.

Example: Let's say John and Jane are married and both earn $50,000 per year. If they file their taxes jointly, their combined income of $100,000 puts them in a higher tax bracket, resulting in a higher tax liability than if they filed as single individuals. This is the marriage penalty.

Another example could be if one spouse has a high income and the other has no income or a low income. When they file jointly, the high earner's income pushes them into a higher tax bracket, resulting in a higher tax liability than if they filed separately.

The marriage penalty can discourage couples from getting married or incentivize them to file separately, which can have other financial implications.