Presumably the GP is referring to the "marriage penalty/bonus" phenomenon that can happen due to filing status for certain people, especially dual income joint

Presumably the GP is referring to the "marriage penalty/bonus" phenomenon that can happen due to filing status for certain people, especially dual income joint filing households vs. single income joint filing households [1]. States with a higher proportion of dual similar-income joint filers with fewer children might hit the penalty more often (like a 2 doctor or lawyer household), and states with higher proportions of single or highly disparate income joint filing households (with a stay-at-home or marginally employed spouse) with more children would hit the bonus more often. But as you mention that effect is usually dominated by the effect of personal exemptions, and not by the fact that tax bracket thresholds between individual and joint filers don't line up perfectly while normalizing for number of filers. Correction: the marriage penalty/bonus has more to do with the disparity in spousal income than magnitude. 1. https://www.investopedia.com/terms/m/marriage-penalty.asp https://www.investopedia.com/terms/m/marriage-penalty.asp