Final answer:
Indifference curves are steeper on the left and flatter on the right due to diminishing marginal utility. The slope of the curve changes based on the starting point's marginal utility of goods consumed. As a person consumes more of a good, the marginal utility from each additional unit becomes lower.
Step-by-step explanation:
Indifference curves like Um are steeper on the left and flatter on the right due to diminishing marginal utility, which means that as a person consumes more of a good, the marginal utility from each additional unit becomes lower.
For example, let's compare two different choices along an indifference curve Um: the choice between A and B, and between C and D. In both choices, Lilly consumes one more book, but between A and B her consumption of doughnuts falls by 36 and between C and D it falls by only five. The reason for this difference is that points A and C are different starting points, and thus have different implications for marginal utility.
At point A, Lilly has few books and many doughnuts. Thus, her marginal utility from an extra book will be relatively high while the marginal utility of additional doughnuts is relatively low. At point C, however, Lilly has many books and few doughnuts. From this starting point, her marginal utility gained from extra books will be relatively low, while the marginal utility lost from additional doughnuts would be relatively high. In short, the slope of the indifference curve changes because of the diminishing marginal utility of both goods.