A Level explanation

Marginal utility (MU) is the extra satisfaction gained from consuming one more unit of a good. The MU curve slopes downward because of the law of diminishing marginal utility: as consumption rises, each additional unit adds less extra satisfaction than the previous one.

Total utility (TU) is the cumulative satisfaction from all units consumed so far. While MU is positive, TU is still rising, but at a decreasing rate. When MU reaches zero, TU is at its maximum — the consumer is satiated.

When MU becomes negative, extra consumption reduces total utility: the consumer would be better off stopping at the peak TU point. In exam answers, link this to rational consumers equating marginal benefit with marginal cost, and why demand curves slope downward.

Essay application (Edexcel A Level)

• Theme 1 — consumer behaviour: explain why demand curves slope downward using diminishing MU; support rational choice and utility maximisation arguments.
• 10–15 mark questions: draw TU/MU to show rising then falling total satisfaction — label where MU = 0 (maximum TU) and where MU turns negative.
• Demerit goods & overconsumption: argue consumers may keep consuming past the optimal point if they ignore marginal analysis (habits, addiction, imperfect information).
• 25-mark evaluation: use the diagram when judging policies that change consumption (e.g. sugar tax, nudges, subsidies) — compare private marginal benefit with social marginal benefit at the margin.
• Link to elasticity: as MU falls, consumers are less willing to pay for extra units — connects micro diagrams to downward-sloping demand in longer essays.