Fundamentals of Business Economics and Management · Theory of Demand and Supply

Law of Demand and Its Exceptions Explained

Updated 10 October 2026 · Fact-checked

The law of demand says that, other things remaining the same, when the price of a good falls its quantity demanded rises, and when price rises quantity demanded falls. The demand curve slopes downward. Exceptions include Giffen goods, Veblen goods, expectations of price change and necessities in emergencies. In MCQs, spot the assumption or the exception.

Understand Law of Demand and Its Exceptions

The law of demand states an inverse relationship between the price of a good and the quantity demanded of it, ceteris paribus (other things remaining equal). If the price of tea falls from ₹20 to ₹10 a cup, you buy more cups. If it rises to ₹30, you buy fewer.

The law holds only when other factors stay constant: consumer income, tastes and preferences, prices of related goods (substitutes and complements), and expectations about future prices. If any of these changes, the whole demand curve shifts. The law is about movement along a curve caused by price alone.

Why does the demand curve slope downward? There are four standard reasons:

Law of diminishing marginal utility: each extra unit gives less satisfaction, so you will pay only a lower price for more units.
Income effect: when price falls, your real income (purchasing power) rises, so you can buy more.
Substitution effect: when a good becomes cheaper relative to its substitutes, you switch to it.
New buyers and extra uses: a lower price brings in new consumers and lets existing buyers put the good to more uses.

Exceptions are cases where a higher price goes with a higher quantity demanded, so the curve slopes upward. A Giffen good is an inferior good that takes a large share of a poor household's budget, with no close substitute. When its price rises, the household cannot afford better food, so it buys more of the inferior good. Coarse grain is the classic example. A Veblen good (conspicuous consumption) is a luxury such as a costly designer watch or jewellery. People buy it for status, so a higher price makes it more attractive.

Other commonly listed exceptions: expectation of a further price rise (people buy more now), expectation of a fall (people wait), necessities in an emergency or scarcity, ignorance (people judge quality by price), and speculative markets such as shares. Remember that these are exceptions to the law, not proof that it is wrong in normal cases.

Key formulas to remember

Law of demand: Price ↑ → Quantity demanded ↓; Price ↓ → Quantity demanded ↑ (other things equal); Inverse relationship. The demand curve slopes downward from left to right.
Assumptions (ceteris paribus): Income, tastes, prices of related goods, expectations and number of buyers are constant; If any of these changes, the curve shifts instead of the law being tested.
Reasons for downward slope: Diminishing marginal utility + income effect + substitution effect + new buyers/extra uses; Learn these four as a set.
Main exceptions: Giffen goods, Veblen goods, expected price changes, emergency necessities, ignorance, speculation; In these cases the curve may slope upward.

How to solve Law of Demand and Its Exceptions questions

Use this method for any theory or scenario MCQ on the law of demand.

1Read the question and decide: is it asking for the law, a reason for the slope, an assumption, or an exception?
2Check whether only price is changing. If income, taste or other factors change, it is a shift, not the law.
3If a scenario shows price and quantity moving in the same direction, look for an exception.
4Identify the good: inferior with large budget share and no substitute means Giffen; prestige luxury means Veblen.
5Check for expectation clues such as 'fears prices will rise further'.
6Eliminate options that mix up the causes, for example calling a Giffen good a luxury.
7Choose the option that matches the exact definition.

Quickest way: Same direction? Name the exception

When to use it: For scenario MCQs where price and quantity are given.

Price and quantity move in opposite directions: the law applies, no exception.
Same direction and the good is a cheap staple used by the poor: Giffen.
Same direction and the good is a status luxury: Veblen.
Same direction because buyers expect future prices: expectation exception.
Question mentions 'ceteris paribus': answer relates to assumptions.

Common mistakes in Law of Demand and Its Exceptions

Treating a shift of the demand curve as a violation of the law.
Students see quantity rise after an income increase even though price did not fall.
Fix: The law is only about price changes with other factors constant. Other changes shift the curve.
Saying Giffen goods are luxury goods.
Giffen and Veblen are learned together and get mixed up.
Fix: Giffen is inferior and staple. Veblen is a luxury bought for status.
Writing that every inferior good is a Giffen good.
Both words appear together in textbooks.
Fix: A Giffen good is a special inferior good with a large budget share and no close substitute. Most inferior goods obey the law.
Forgetting the ceteris paribus assumption.
Students memorise only 'price up, demand down'.
Fix: Always attach 'other things remaining equal' to the statement.
Mixing up quantity demanded with demand.
Both terms sound alike.
Fix: A price change moves quantity demanded along the curve. Changes in other factors change demand, shifting the curve.

Worked examples

Example 1

A poor household spends most of its income on a coarse grain, which has no close substitute. When the price of the grain rises, the household buys more of it. This is an example of: (A) Veblen good (B) Giffen good (C) Normal good (D) Luxury good

Show the solution

Price rises and quantity demanded also rises, so the law of demand seems violated.
The good is a cheap staple for a poor household, with no close substitute and a large share of the budget.
These are the features of a Giffen good, which is an inferior good.
Veblen goods are status luxuries, normal and luxury goods obey the law.

Answer: (B) Giffen good

Example 2

Which of the following is NOT a reason why the demand curve slopes downward? (A) Income effect (B) Substitution effect (C) Law of diminishing marginal utility (D) Buying a good for prestige at a higher price

Show the solution

List the standard reasons: income effect, substitution effect, diminishing marginal utility, new buyers and extra uses.
Options A, B and C are on this list.
Option D describes Veblen behaviour, where a higher price attracts buyers. That is an exception, not a reason for the downward slope.

Answer: (D) Buying a good for prestige at a higher price

Exam tips

Questions often ask you to pick the exception from four options. Learn Giffen and Veblen definitions word for word.
Expect a 'which is NOT' question on reasons for the downward slope.
Watch for scenarios where income or taste changes. The answer is a shift, not the law.
Since there is no negative marking, always attempt every question and eliminate options first.

Practice questions from Theory of Demand and Supply

Law of Demand and Its Exceptions in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Law of Demand and Its Exceptions: frequently asked questions

What is the law of demand in simple words?

When the price of a good falls, people buy more of it, and when the price rises, they buy less, assuming nothing else changes. This gives a downward-sloping demand curve.

What is the difference between Giffen goods and Veblen goods?

A Giffen good is an inferior staple that poor households buy more of when its price rises. A Veblen good is a prestige luxury whose higher price increases its appeal to status-seeking buyers.

Why does the demand curve slope downward?

Mainly because of diminishing marginal utility, the income effect and the substitution effect. A lower price also attracts new buyers and creates extra uses for the good.

What are the assumptions of the law of demand?

The law assumes that income, tastes and preferences, prices of related goods, expectations and the number of buyers stay constant. Only the price of the good changes.