More income equals more tax. In fact, at $12,000 income you had close to zero tax. so the next 12,000 is fully taxed.
That truth should be self evident, but the tax system has been made complicated by the Earned Income Credit, a government give away.
The basic EIC works on a
"bell curve," rising as a worker's wages rise reaching a maximum when
annual earnings are between $14,000 and $18,350 (Single with 2 or 3 children
2018) and then declining gradually until it phases out altogether. If your
income is on the up slope of the EIC curve more earned income will increase your
EIC but if your income is on the down slope, more income (of any kind) will
reduce you EIC. The “upslope" on the EIC curve is very steep; basically
for every $3 you earn, the government gives you another dollar. See the curve (graph)
at:
http://www.taxpolicycenter.org/briefing-book/key-elements/family/eitc.cfm
The earned income credit is first calculated (actually looked up
in a table) on your earned income then it is calculated on your total income (AGI).
You get the lesser of the two calculated EIC numbers. See the 2018 EIC table
at:
https://www.irs.gov/pub/irs-pdf/p596.pdf
