Marginal Rate of Substitution, or MRS, measures the amount of one good that a consumer can sacrifice in order to gain an additional unit of another good while maintaining the same level of satisfaction. For example, if the MRS of books for movie tickets is 2, it means that the consumer is willing to sacrifice two movie tickets to obtain one additional book in order to maintain equal satisfaction.

The downward slope of the indifference curve is due to diminishing MRS. This is because the consumer's marginal rate of utility of consuming any good declines as the consumption of that good is increased, the consumption of another good to make up for the one unit loss of the first good should be decreased for the consumer to remain on the same level of satisfaction.

Diminishing MRS

The MRS changes along the indifference curve. In the example of books and movie tickets, as the consumer acquires more books, they are willing to give up fewer and fewer movie tickets for each additional book. This is because the fewer movies the consumer watches, the more valuable each movie becomes. This means the MRS is diminishing.