The Theory of Consumer Behaviour

Utility simply means the satisfaction or pleasure you get from consuming goods or services. Think of it like this: when you buy a plate of jollof rice when you're hungry, that satisfaction you feel is utility. There are three main types to understand. Total utility is the complete satisfaction from consuming all units of a product—eating five pieces of meat gives you more total satisfaction than eating one. Marginal utility is the extra satisfaction from consuming one additional unit—that fifth piece of meat brings less satisfaction than your first piece because you're getting fuller. This is why the law of diminishing marginal utility exists: each extra unit gives less satisfaction than the previous one.

The law of demand states that when the price of a good falls, people want to buy more of it, and when the price rises, they buy less. This relationship between price and quantity demanded is inverse—they move in opposite directions.

Think about bread in Nigeria. When the price of a loaf drops from ₦300 to ₦200, families buy more bread because it's now affordable. But if the baker increases the price to ₦400, many households reduce their purchases and look for alternatives. This happens because people have limited money and naturally buy more when things cost less.

Understanding this law helps you answer questions about market behaviour and predict what happens when prices change. You'll see it tested in price-quantity scenarios and real-life market situations.

Marginal utility is the extra satisfaction you get from consuming one additional unit of a good. Think of it this way: eating your first plate of jollof rice at a party gives you tremendous happiness. The second plate still tastes good but gives you less additional satisfaction. By the fourth plate, you might feel stuffed and get almost zero satisfaction—that's marginal utility in action.

This concept explains why we eventually stop buying more of something even when we have money. Your satisfaction decreases with each extra unit consumed. Economists call this the law of diminishing marginal utility, and it's why prices must fall to encourage you to buy more items.

Understanding this helps explain consumer choices and demand curves on your UTME questions.

An indifference curve shows different combinations of two goods that give a consumer equal satisfaction. Think of it like this: if you're buying rice and beans, your indifference curve might show that 10kg of rice with 2kg of beans gives you the same happiness as 6kg of rice with 4kg of beans. You're indifferent between these combinations because your total satisfaction stays the same.

Marginal utility is the extra satisfaction you get from consuming one additional unit of a good. When you buy your first plate of jollof rice, the satisfaction is high. The second plate gives less extra satisfaction, and the third even less. This is why indifference curves slope downward—as you consume more of one good, you need less of the other to maintain equal satisfaction.

The key principle here is diminishing marginal utility: each extra unit brings less joy than the previous one.

When a good's price falls, two things happen in your wallet. The substitution effect means you buy more of that item because it's now cheaper compared to others—you substitute it for pricier alternatives. The income effect means the price drop makes you feel richer, so you can afford more of everything, including that good.

Picture this: rice prices drop in Lagos. You buy more rice because it's cheaper than beans now (substitution effect). But you also feel wealthier since your money stretches further, so you buy more rice anyway (income effect). Both effects push you toward buying more rice.

For normal goods, both effects work together, increasing quantity demanded. For inferior goods like garri, the income effect might reduce demand if you buy less when you feel richer, but substitution usually wins.

Consumer surplus is simply the difference between what you're willing to pay for something and what you actually pay. Think of it as the gain or benefit you get from a purchase. When you buy something cheaper than you expected, that extra satisfaction is your consumer surplus.

Let's use a relatable example. Imagine you go to the market willing to pay ₦5,000 for a new school bag, but you find it selling for only ₦3,000. That ₦2,000 difference is your consumer surplus—you've saved money while getting exactly what you wanted. The seller benefits too because they made a sale. This happens constantly in Nigerian markets and online shopping platforms where prices vary.

Understanding consumer surplus helps explain why people prefer certain markets and why demand curves slope downward. Consumers always seek better value.