What are three concepts that explain why demand curves slope downward?

1) This law of diminishing marginal utility assumes that when more goods or services are consumed, the marginal benefit (utility) for the consumer falls, therefore, consumers are willing to pay less.
2) Consider two normal goods – x and y, suppose that the price of good x increases.
An increase in the price of good x encourages the consumer to buy less of x and more of y by substituting the increase in the price of x reduces the consumer’s real income or purchasing power. Since both goods are normal, the demand for both goods falls with the fall in real income. Since both effects lead to a fall in demand for good x, we get a downward demand curve.
Now suppose a good x is worse. Consequently, the demand for product x increases with the fall in real incomes of the population.
If the substitution effect outweighs the income effect, then the demand for good x falls as the price rises, resulting in a downward demand curve; if the income effect exceeds the substitution effect, then the demand for x increases with the price, resulting in an upward sloping demand curve.

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