TU と MU の関係

Total utility is the overall satisfaction from consuming all units of a product, while marginal utility measures the additional satisfaction from consuming one more unit. For example, Nicole's total utility from two pizza slices is the combined satisfaction from both slices, whereas her marginal utility from the second slice is only the extra satisfaction that slice provides.

Marginal utility is calculated by dividing the change in total utility by the change in quantity consumed. If total utility increases by 10 units when consumption rises from two to three pizzas, the marginal utility of the third pizza is 10 units. This calculation reveals how much additional satisfaction each new unit provides.

The slope of the total utility curve represents marginal utility. When the slope is steep and positive, marginal utility is high. As the curve flattens, marginal utility decreases. When the curve peaks and begins declining, marginal utility becomes negative, indicating that additional consumption reduces overall satisfaction.

Total utility is maximized at the point where marginal utility equals zero. This occurs where the marginal utility curve intersects the x-axis. At this point, consuming additional units provides no additional satisfaction. Beyond this point, marginal utility becomes negative and total utility begins to decrease.

As long as marginal utility remains positive, total utility increases with each additional unit consumed. When marginal utility reaches zero, total utility stops increasing and reaches its maximum. Once marginal utility becomes negative, total utility declines with further consumption, reflecting diminishing satisfaction from additional units.

The total utility curve flattens or declines because marginal utility diminishes as consumption increases. Initially, each additional pizza slice provides significant satisfaction, but as Nicole eats more, each new slice adds less satisfaction. Eventually, additional slices may cause discomfort, making marginal utility negative and reducing total satisfaction.

The marginal utility curve is derived from the total utility curve and is drawn on a separate graph. The slope of the total utility curve at any point equals the marginal utility at that quantity. When total utility rises steeply, marginal utility is high; when it flattens, marginal utility approaches zero; when it declines, marginal utility becomes negative.