According to the standard definition of a normal good in microeconomics, the demand for such a good tends to decrease when there is a ________ in the consumer’s income, other things remaining the same.

Introduction / Context:
Normal and inferior goods are classified based on how demand responds to changes in consumer income. For a normal good, demand moves in the same direction as income, whereas for an inferior good, demand moves in the opposite direction. This question tests your understanding of what happens to the demand for a normal good when the consumer’s income changes.

Given Data / Assumptions:
- The good in question is explicitly a normal good.
- Other factors such as prices and tastes are assumed to remain constant (ceteris paribus).
- The question asks when demand for the normal good decreases in relation to income changes.
- Options mention increase, decrease, constant level and doubling of income.

Concept / Approach:
By definition, for a normal good, the income effect is positive: when income rises, quantity demanded increases; when income falls, quantity demanded decreases. This is because consumers generally buy more of goods they view as desirable when they have more purchasing power. Therefore, we are looking for the type of income change that would cause demand to fall, which must be a decrease in income.

Step-by-Step Solution:
Step 1: Recall the definition of a normal good: demand increases when income increases and decreases when income decreases.Step 2: Identify which option describes an income movement that should cause demand to decrease for a normal good.Step 3: Option B, decrease in income, clearly matches this: when income goes down, consumers cut back on normal goods.Step 4: Option A, increase in income, is associated with higher demand, not lower demand, for a normal good.Step 5: Option C, constant income, implies no change in demand due to income, and option D, doubling income, would also lead to higher demand, not lower, for a normal good.

Verification / Alternative check:
You can verify with a simple example: consider a normal good such as high quality clothing. When a person’s income rises, they are likely to buy more or better clothing. When their income falls due to job loss or salary cuts, they reduce such purchases. This pattern is consistent with the definition of a normal good and confirms that a decrease in income leads to a decrease in demand.

Why Other Options Are Wrong:
Option A is wrong because income increases lead to increased demand, not decreased demand, for normal goods. Option C is wrong because if income remains constant, there is no reason for demand to change due to income alone. Option D is wrong because doubling income is an extreme form of increase, which would again increase demand for normal goods.

Common Pitfalls:
Some students confuse normal goods with inferior goods, where demand moves inversely with income. For inferior goods, a rise in income can reduce demand, as consumers switch to higher quality alternatives. To avoid this confusion, remember that normal means moving in the normal or same direction as income, whereas inferior means moving in the opposite direction.

Final Answer:
For a normal good, demand decreases when there is a decrease in the consumer’s income.