Causes of a shift in the budget line

📚 Causes of a Shift in the Budget Line

What is a Budget Line?

A budget line shows all the combinations of two goods that a consumer can buy with a fixed amount of money.

If the prices of good X and good Y are \(px\) and \(py\) and the income is \(I\), the budget line is given by:

\$\$

px\,x + py\,y = I

\$\$

It’s a straight line with a slope of \(-\dfrac{px}{py}\). Think of it as a straight road you can travel along with your money – the farther you go in one direction, the less you can go in the other.

When Does the Budget Line Shift?

Three main factors can shift the whole line:

  1. Change in Income (I) – More or less money to spend.

  2. Change in the Price of Good X (px) – A price drop or rise.

  3. Change in the Price of Good Y (py) – A price drop or rise.

Exam Tip: When answering, always state the direction of the shift (to the right or left) and explain whether the slope changes. Use the word “shift” for changes in income or prices, not “tilt” (which refers to slope changes).

Illustrative Examples

  • Income Increase: If a student gets a scholarship, the budget line moves outward (to the right), allowing more of both goods.

  • Price Drop of Good X: If the price of pizza falls, the budget line pivots outward on the X‑axis, making pizza cheaper relative to the other good.

  • Price Rise of Good Y: If the price of textbooks rises, the budget line pivots inward on the Y‑axis, reducing the quantity of textbooks that can be bought.

Visualising the Shift

ScenarioBudget Line ChangeSlope Effect
Income ↑Outward shift (right)No change
\(p_x\) ↓Outward pivot on X‑axisSlope becomes less steep (more negative)
\(p_y\) ↑Inward pivot on Y‑axisSlope becomes steeper (more negative)

Exam‑Ready Checklist

  • Label the axes: \(x\) for Good X, \(y\) for Good Y.

  • Show the equation of the original budget line.

  • Indicate the direction of the shift with an arrow.

  • Explain whether the slope changes and why.

  • Use the word “shift” for income or price changes.

Quick Recap with Emojis

💰 Income Change: Line moves right or left.

📉 Price Drop: Line pivots outward.

📈 Price Rise: Line pivots inward.

🧩 Remember: Shift = movement of the whole line; Tilt = change in slope.