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To get a better estimate of the influence of income on consumption, we need to take out any influence that flows from consumption to income. A bit of mathematics and analysis that can wait for more advanced courses can solve this feedback problem. When it is solved, the relationship between consumption and disposable income is weakened only a little. Disposable income remains the dominant determinant of consumption spending, but it is not the only thing that can matter.
Ignoring feedback led many economists to emphasize that the correlation between consumption and income resulted solely from effects of income on consumption. As a result, their estimations of the simple consumption function gave results that looked stronger than they actually were. In contrast, ignoring feedback between investment and income led many economists to treat the correlation here as solely the result of effects of investment on income. To many of these economists, a simple income-expenditure model largely explained how the world worked. Autonomous (and thus unexplainable) changes in investment would change income, and these changes would be multiplied by the feedback between income and consumption. If, however, one can explain changes in investment by changes in income or financial conditions, then the simple income-expenditure model will be a partial and perhaps misleading way to view the world.
The IS curve is built assuming that business investment should be sensitive to both interest rates and expectations of future earnings. In addition, investment includes residential construction and changes in inventories. These forms of investment respond to financial conditions and total spending, though not in the same ways as business investment. As with consumption, changes in investment may react to changes in the economy with a time lag.
Changes in income can also affect investment through the accelerator principle. In 1939 Paul Samuelson published an article that showed how the accelerator could be combined with the income expenditure model to give a dynamic model. In this model investment causes changes in income and in turn changes in income (or consumption, depending on how the model is structured) cause changes in investment.