The Law of Diminishing Returns states that as you add more of one input to a fixed set of other inputs, the marginal output from each additional unit eventually decreases. After a certain point, each extra unit of input produces less additional output than the previous unit. This fundamental economic principle governs hiring decisions, marketing spend, feature development, workout volume, and most resource allocation decisions.
Opportunity Cost is the value of the best alternative forgone when a decision is made. It is one of economics' most fundamental concepts and one of the most consistently ignored in practical decision-making. Every choice eliminates other choices; the opportunity cost is the best of those eliminated alternatives. Failing to account for opportunity cost leads to systematic overvaluation of existing commitments, undervaluation of alternatives, and poor allocation of time, money, and attention.