[CaptainNegative]

The key problem for <$20k earners is the constant 7.65% FICA ("Social Security/Medicare") Tax plus any local taxes, not income tax.

After the standard deduction, the net income tax rate for a single filer making $20k is 3.75%. For a couple filing jointly it's 0 up through $25.5k/yr.

In both cases, the EITC guarantees those with at least one child a (substantially) negative effective tax rate, while single child-free filers are left with about a 2.6% effective rate.

[SomeHacker44]

$20k x 2 >> $25.5k. Marriage penalty in action.

[CaptainNegative]

The post-deduction income tax rate for a MFJ household making $40k is also 3.75%. There are some examples of a marriage penalty -- and others of a marriage benefit -- but this isn't an example of either one of them.