Full and Haig–Simons income "Full income" refers to the accumulation of both the monetary and the non-monetary consumption-ability of any given entity, such as a person or a household. According to what the economist
Nicholas Barr describes as the "classical definition of income" (the 1938 Haig–Simons definition): "income may be defined as the... sum of (1) the market value of rights exercised in consumption and (2) the change in the value of the store of property rights..." Since the consumption potential of non-monetary goods, such as leisure, cannot be measured, monetary income may be thought of as a proxy for full income. In
consumer theory 'income' is another name for the "budget constraint", an amount Y to be spent on different goods x and y in quantities x and y at prices P_x and P_y. The basic equation for this is :Y=P_x \cdot x + P_y \cdot y This equation implies two things. First buying one more unit of good x implies buying \frac{P_x}{P_y} less units of good y. So, \frac{P_x}{P_y} is the
relative price of a unit of x as to the number of units given up in y. Second, if the price of x falls for a fixed Y and fixed P_y, then its relative price falls. The usual hypothesis, the
law of demand, is that the quantity demanded of x would increase at the lower price. The analysis can be generalized to more than two goods. The theoretical generalization to more than one period is a multi-period
wealth and income constraint. For example, the same person can gain more productive skills or acquire more productive income-earning assets to earn a higher income. In the multi-period case, something might also happen to the economy beyond the control of the individual to reduce (or increase) the flow of income. Changing measured income and its relation to consumption over time might be modeled accordingly, such as in the
permanent income hypothesis. ==Legal definitions==