Keynesian Cross
In the "Keynesian cross diagram," a desired total spending (or aggregate expenditure, or "aggregate demand") curve is often drawn as a rising level of D as total national output and income rise. This increase is due to the positive relationship between C and consumers' disposable income in the consumption function. It may also rise due to increases in investment (due to the accelerator effect), while this rise is reduced if imports and tax revenues rise with income. Equilibrium in this diagram occurs where total spending (D) equals the total amount of national income (which corresponds to total national output or production). Here, a total demand equals total supply.
In the diagram, the equilibrium level of output, income, and demand is determined where this desired spending curve intersects the "Z curve," a line that represents the equality of total income and output. This is at point E, determining the equilibrium levels of output and income on the horizontal axis (where the arrow points).
The movement toward equilibrium is mostly via changes in inventories inducing changes in production and income. If current output exceeds the equilibrium, inventories accumulate, encouraging businesses to cut back on production, moving the economy toward equilibrium. Similarly, if the level of production is below the equilibrium, then inventories run down, encouraging an increase in production and thus a move toward equilibrium. This equilibration process occurs when the equilibrium is stable, i.e., when the D line is steeper than the Z line.
The equilibrium level of output determines the equilibrium level of employment in the model. (In a dynamic view, these are connected by Okun's Law.) There is no reason within the model why the equilibrium level of employment should correspond to full employment. Bringing in other considerations may imply this correspondence, though.
If any of the components of aggregate demand (C + Ip + G + NX) rises at each level of income, for example because business becomes more optimistic about future profitability, that shifts the entire D line upward. This raises equilibrium income and output. Similarly, if the elements of D fall, that shifts the line downward and lowers equilibrium output. (The Z line does not shift under the definition used here.)
In the diagram, the equilibrium level of output, income, and demand is determined where this desired spending curve intersects the "Z curve," a line that represents the equality of total income and output. This is at point E, determining the equilibrium levels of output and income on the horizontal axis (where the arrow points).
The movement toward equilibrium is mostly via changes in inventories inducing changes in production and income. If current output exceeds the equilibrium, inventories accumulate, encouraging businesses to cut back on production, moving the economy toward equilibrium. Similarly, if the level of production is below the equilibrium, then inventories run down, encouraging an increase in production and thus a move toward equilibrium. This equilibration process occurs when the equilibrium is stable, i.e., when the D line is steeper than the Z line.
The equilibrium level of output determines the equilibrium level of employment in the model. (In a dynamic view, these are connected by Okun's Law.) There is no reason within the model why the equilibrium level of employment should correspond to full employment. Bringing in other considerations may imply this correspondence, though.
If any of the components of aggregate demand (C + Ip + G + NX) rises at each level of income, for example because business becomes more optimistic about future profitability, that shifts the entire D line upward. This raises equilibrium income and output. Similarly, if the elements of D fall, that shifts the line downward and lowers equilibrium output. (The Z line does not shift under the definition used here.)