Changes in price can affect buyers' purchasing decisions; this effect is called the income effect. Increases in price, while they don't affect the amount of your paycheck, make you feel poorer than you were before, and so you buy less. Decreases in price make you feel richer, and so you may feel like buying more.
What if we're looking at two goods at once? For instance, a fast food chain sells hamburgers and hot dogs. If the price of hamburgers goes up, but the price of hot dogs stays the same, you might be more inclined to buy a hot dog. This tendency to change your purchase based on changes in relative price is called the substitution effect. When the price of hamburgers goes up, it makes hamburgers relatively expensive and hot dogs relatively cheap, which influences you to buy fewer hamburgers and more hot dogs than you usually would. Likewise, a decrease in hamburger price would cause you to eat more hamburgers and fewer hot dogs, according to the substitution effect.
The income effect also affects buying decisions when there are two (or more) goods. When the price of hamburgers goes up, it makes you feel relatively poorer, so your tendency might be to buy fewer of both hamburgers and hot dogs.
If you look at the combined results of the income effect and the substitution effect, the total effect is a little unclear. According to the income effect, an increase in the price of hamburgers decreases consumption of both hamburgers and hot dogs. According to the substitution effect, however, hamburger consumption drops, but hot dog consumption rises. Thus, while it is clear what happens to hamburger consumption, since both effects tend to cause a decrease, we cannot be sure what happens to hot dog consumption, since there is both an increase (substitution effect) and a decrease (income effect).
While we cannot be absolutely certain about the net result, in general, the substitution effect is stronger than the income effect. That is, when the price of hamburgers goes up, you will most likely eat fewer hamburgers and more hot dogs, since the change in relative prices (substitution effect) affects you more than the perceived change in your income (income effect).
Another factor influencing demand is one which marketers and advertisers are always trying to understand and target: buyers' preferences. What do people like? When and how do they like it? Still looking at soda, it makes sense that people drink more soda when it's hot, or when they're eating a meal, or when they've been exercising. In these cases, buyers' preferences have changed: they want the soda more, and are therefore willing to pay more for the same good. Likewise, if it's snowing, fewer people will crave a cold soda, and the price they are willing to pay for a cold soda is lower, although they may be willing to pay a little extra money for a hot coffee.
Are all goods the same? Is more always better? Up to this point, we have been assuming that when we have more money, or feel like we have more money, we will tend to buy more goods. It makes sense: the more money we have, the more we buy. If we have less money, or if the price goes up, however, we tend to buy less. Because this is usually the case, we call such goods normal goods. If you buy more of a good when you have more money, that good is a normal good. If the price of a normal good increases, you buy less.
There are some exceptions, however: not all goods are normal goods. For instance, if an increase in your income causes you to buy less of a good, that good is called an inferior good. For instance, "poor college students" often satisfy themselves with generic soda and cheap ramen. When they get jobs and a steady income, however, they might forego the cheap soda and ramen in favor of Coke and pasta. In this example, the generic soda and cheap ramen are inferior goods.
Income and substitution effects change demand differently with different types of goods. For instance, we have been looking at income and substitution effects when a buyer is faced with a choice between two normal goods. An increase in the price of good A will cause a decrease in consumption of A, and an increase in consumption of good B (assuming that the substitution effect is stronger than the income effect). If good A is a normal good, and good B is inferior, however, the results will be different.
Why is this true? Consider the case where the price of good A goes up.
If the A is still normal and B is still inferior, and the price of A falls, then the substitution effect will cause higher consumption of A and lower consumption of B, and the income effect will cause higher consumption of A and lower consumption of B. Because the buyer now feels richer, they are less inclined to buy the inferior good.
Another exception is the case where an increase in price causes an increase in demand. This results in an upward-sloping demand curve, and the good is called a Giffen good. Giffen goods are theoretically possible, but very improbable, since it is unlikely that an increase in price causes increase in demand. One possible justification for a Giffen good is that people associate higher prices with status, luxury, and quality, so that a higher price might increase the perceived value of a good. In reality, however, this effect is outweighed by the overwhelming tendency to prefer lower prices: even if a few people prefer the added cachet of a high-priced luxury good, the general public will prefer lower prices. Another possible case that could cause a Giffen good is the case in which a good is inferior and the income effect outweighs the substitution effect. To illustrate, assume that ACME Cola is an inferior good. When it's price increases, the income effect makes Calvin feel poorer. If the income effect is very strong, and the substitution effect is very weak, then Calvin will buy more ACME Cola, because the consumption of inferior goods increases with decreases in income. This, too, is unlikely, however, because the substitution effect is almost always stronger than the income effect.