The belief that a pay rise can leave someone worse off through tax is widespread and usually wrong. It comes from misreading how banded systems apply rates.
Bands apply to slices, not to totals
A progressive income tax divides income into bands and applies a different rate to each. Income within a band is taxed at that band's rate and no other.
Crossing into a higher band therefore affects only the portion above the threshold. Everything below continues to be taxed exactly as before.
The average rate paid across all income is consequently lower than the marginal rate, and rises gradually rather than jumping at each boundary.
Marginal and average answer different questions
The marginal rate describes what happens to the next unit earned, which is the relevant figure for decisions about overtime, a second job or additional dividends.
The average rate describes the total burden across all income, which is the relevant figure for comparing overall tax positions between years or between people.
Confusing the two produces the impression that a large share of total income disappears, when in fact only the top slice is taxed at the highest rate.
Genuine cliffs exist but sit elsewhere
While income tax bands themselves are gradual, other parts of a tax and benefit system can withdraw an allowance or a payment abruptly at a threshold.
Where a benefit is lost entirely at a fixed income level, an extra unit of earnings really can reduce net income. These are design features of specific provisions rather than of the rate structure.
Such thresholds vary by jurisdiction and change over time, which is why general reasoning about them is unreliable and the current rules must be checked.
Tapered allowances create high effective rates
Some systems withdraw a personal allowance gradually as income rises, so each additional unit of income both attracts tax and reduces the untaxed amount.
The combined effect produces an effective marginal rate over that income range that exceeds the headline rate of the band it sits within.
These zones are rarely advertised and are the main reason a marginal rate calculated from the published bands alone can be substantially wrong.
Why the distinction matters in practice
Decisions about additional work, pension contributions or the timing of income all depend on the marginal rate, including the effect of any taper or threshold nearby.
Because the relevant figure depends on total income and on provisions outside the rate table, it cannot be read off a summary of bands.
Rules differ substantially between jurisdictions and are revised regularly, so this is an area where current official guidance or a professional is the appropriate source.