Comprehensive Study Notes on Demand and the Law of Demand

Core Principles and Meaning of Demand

A consumer does not demand a product simply because they want it. Demand exists only when a desire is backed by both the willingness and the ability to purchase the item at a specific price during a specific time period. This distinction is vital: desire is a mere wish to possess something, whereas demand is an effective desire. Effective desire is calculated as the sum of desire, willingness to buy, and the ability to pay, all stated in the context of price and time.

To identify demand clearly, four specific criteria must be met. First, the consumer must be willing, meaning they actually want to buy the specific item, such as a pair of shoes. Second, they must be able, meaning they possess the necessary purchasing power through money or credit. Third, a specific price must be attached, as demand levels change significantly depending on whether an item costs $\text{₹500}$ or $\text{₹900}$. Finally, demand is a flow concept, meaning it is measured over a specific period, such as two liters of milk per day or ten notebooks per month, rather than as a single isolated event.

Classifications and Types of Demand

Demand is categorized into several types based on who is buying, the timing of the intent, and how the goods are used. Individual or household demand refers to the quantity a single consumer or family is willing to buy at different prices over a period. Market demand is the total sum of all individual demands within a city or region at those various prices.

Ex ante demand represents the planned or desired quantity a buyer intends to purchase before the transaction happens, such as a family planning to buy $5\,kg$ of rice. Ex post demand is the actual quantity finally purchased, which might be lower, such as $4\,kg$, if stock was limited.

In terms of product relationships, joint demand occurs when two or more goods are used together to satisfy a want, such as a car and petrol or bread and butter. Competitive or substitute demand occurs between goods that compete for the same want, like tea and coffee. Direct demand applies to final goods that satisfy wants immediately, like clothes or milk. Derived demand arises when the demand for one good is a result of the demand for another; for example, the demand for steel or cement rises because of the demand for house construction. Composite demand occurs when a single commodity has multiple uses, such as electricity, coal, or milk being used for various different purposes.

The Demand Function and Its Determinants

The demand function expresses the functional relationship between the demand for a product and its various determinants. It is written as:

$D_n = f(P_n, P_r, Y, T, E, H, \bar{Y}, G, \dots)$

In this function, $D_n$ is the demand for commodity $n$ (the dependent variable), while the independent variables include $P_n$ (the price of commodity $n$), $P_r$ (prices of related commodities), $Y$ (consumer income), $T$ (tastes and preferences), $E$ (expectations), $H$ (population size), $\bar{Y}$ (distribution of income), and $G$ (government policy).

There are 11 primary determinants that influence demand levels:

1. Price of the Commodity: Generally, as price falls, quantity demanded increases, and as price rises, quantity demanded decreases.
2. Income of the Consumer: For normal goods, an increase in income leads to an increase in demand. However, for inferior goods, demand may fall as income rises because consumers switch to superior substitutes.
3. Tastes and Preferences: Favorable trends, like a fitness craze, can spike demand for related products like gym equipment.
4. Prices of Related Goods: This depends on whether goods are substitutes (where a rise in coffee price raises tea demand) or complements (where a rise in petrol price lowers car demand).
5. Consumer Expectations: If people expect prices to rise in the future, current demand increases.
6. Consumer-Credit Facilities: Easier access to loans increases the demand for expensive durable goods like cars.
7. Demonstration Effect: Consumption is often influenced by imitating the styles or luxury purchases of social circles.
8. Size and Composition of Population: A larger population or a higher concentration of a specific age group (like teenagers) increases market demand for relevant products.
9. Distribution of Income: Unequal distribution can lead to higher demand for luxury goods.
10. Climatic Factors: Seasonal changes dictate demand, such as heaters being demanded in winter and fans in summer.
11. Government Policy: Taxes can reduce demand by raising prices, while government spending can boost demand for materials like construction inputs.

Income-Demand Relationships and Related Goods

Goods are classified by how their demand reacts to changes in consumer income. Normal goods show a direct relationship where both income and demand move in the same direction. Inferior goods, such as coarse grains like maize or jowar, exhibit an inverse relationship beyond a certain income level because consumers can then afford better substitutes. Inexpensive necessities see demand rise with income initially, but consumption eventually becomes constant regardless of further income increases because there is a practical limit to how much of a basic necessity one can use.

Related goods are split into substitutes and complements. Substitutes have a direct relationship: if the price of a substitute rises, the demand for the given good also rises (e.g., tea and coffee). Complements have an inverse relationship: if the price of a complement rises, the demand for the given good falls (e.g., car and petrol). This is known as the cross-price effect.

The Law of Demand and the Demand Curve

The Law of Demand states that, ceteris paribus (all other things remaining constant), the quantity demanded of a commodity increases when its price falls and decreases when its price rises. This inverse relationship is why the demand curve slopes downward from left to right. The law relies on several assumptions, known as the ceteris paribus conditions: consumer income, related goods' prices, tastes, population size, and other determinants must remain unchanged.

A demand schedule is a table showing different quantities demanded at different prices. An individual demand curve is the graphical representation of this table. Market demand is calculated through the horizontal summation or aggregation of all individual demand curves, meaning one adds the quantities demanded by every consumer at each specific price level.

Reasons for the Downward Slope of the Demand Curve

There are five primary reasons why the demand curve is negatively sloped:

1. Law of Diminishing Marginal Utility: As a consumer buys more units, the extra satisfaction from each additional unit decreases, so they are only willing to buy more if the price is lower.
2. Income Effect: When the price of a good falls, the consumer's real income (purchasing power) increases. For example, if income is $\text{₹200}$ and mango prices fall from $\text{₹100}$ to $\text{₹80}$, the consumer has $\text{₹40}$ of extra purchasing power, allowing them to buy more.
3. Substitution Effect: When a commodity becomes cheaper relative to its substitutes, consumers switch from the more expensive substitute to the cheaper commodity.
4. Number of Consumers: A lower price attracts new buyers who could not previously afford the product.
5. Multiple Uses: If a good like milk or electricity has many uses, a lower price allows it to be used for less urgent purposes, increasing total demand.

Exceptions to the Law of Demand

In certain circumstances, demand may rise even if the price rises. These exceptions include:

1. Giffen Goods: These are highly inferior goods that consume a large portion of a person's income. A price fall might actually lower demand because the increase in real income allows the consumer to buy superior food instead.
2. Snob or Status Goods: Items like diamonds or luxury cars may see higher demand at higher prices because of their prestige or display value.
3. Future Price Expectations: If a shortage or further price hike is expected (like petrol before a tax increase), people buy more now despite high prices.
4. Emergencies: During wars, floods, or famines, people buy essentials regardless of high prices due to fear of total unavailability.
5. Veblen Effect: This is the price-quality relationship where consumers assume a higher price is a signal of superior quality.
6. Bandwagon Effect: Demand increases because a product has become a popular trend that everyone else is following.
7. Changes in Fashion: If a product is out of fashion, demand will remain low even if the price is cut drastically.

Changes in Demand versus Changes in Quantity Demanded

It is critical to distinguish between shifts of the curve and movements along the curve. A change in quantity demanded is caused only by a change in the product's own price, resulting in a movement along the same demand curve. This is categorized as expansion (downward movement when price falls) or contraction (upward movement when price rises).

A change in demand is caused by factors other than the product's own price (income, tastes, related prices). This results in a shift of the entire demand curve. An increase in demand is a rightward shift, meaning more is purchased at each price level. A decrease in demand is a leftward shift, meaning less is purchased at each price level. For instance, a rise in income for a normal good causes an increase (shift), whereas a drop in the good's own price causes an expansion (movement).

Questions & Discussion

Q: If a mobile phone becomes $\text{₹5,000}$ cheaper and no other factors change, what occurs? A: This is a change in the commodity's own price, leading to an increase in quantity demanded. This is called expansion of demand and is shown by a downward movement along the existing demand curve.

Q: What happens if a student's income rises and they buy more books at the same price? A: This constitutes an increase in demand because the change was triggered by income, not the price of the books. The demand curve shifts to the right.

Q: Why does tea demand rise when coffee becomes more expensive? A: Tea and coffee are substitutes. As coffee becomes relatively more expensive, consumers switch to tea, causing an increase in tea demand.

Q: How does an emergency like a flood affect the law of demand? A: In an emergency, the law of demand often fails to operate because the fear of shortage drives consumers to purchase essential goods even as prices rise.

Q: Explain the difference between derived and composite demand with an example. A: Derived demand occurs when you want one thing because you need another (e.g., demanding steel because you are building a house). Composite demand occurs when one resource can be used for many different things (e.g., steel being used for cars, utensils, and construction).