>>At one point - I don't think it does any more - filing a joint return had a benefit, reflecting the idea that married people are helping the country.
>
>One of my daughters was on the fence with her partner, whether to get married or not, and the tax was a kind of a tipping point. So they decided to do it around the kid's 2nd birthday. Then they checked again and they'd actually be worse off filing jointly. Now the kid is 5 years old, has a sister, and they aren't married.

Yes.

I suspect that the original law assumed one earner in the household and it was never adjusted when women started working full time.