The government purchases line is commonly presented in one of two forms reflecting autonomous government purchases and induced government purchases. The simplest is a horizontal government purchases line, one with a zero slope, that illustrates autonomous government purchases. With a horizontal line, government purchases are constant for all levels of aggregate income or production, hence autonomous and unaffected by income. A more realistic government purchases line is positively sloped, indicating a degree of induced government purchases. In this case, the vertical intercept, or Y-intercept, of the government purchases line marks autonomous government purchases and the slope represents induced government purchases. Of no small importance, the slope of the government purchases line is also the marginal propensity for government purchases (MPG).

Government Purchases Line

The horizontal red line, labeled G in the exhibit to the right, indicates government purchases that are completely autonomous. There are no induced government purchases indicated by this line. The slope of the government purchases line is zero (MPG = 0). The intercept of this horizontal line indicates autonomous government purchases, which is $2 trillion in this case.

An alternative government purchases line, one with induced government purchases, can be illustrated in the exhibit with a simple click of the [An Induced Line] button. The new red line, labeled G' in the exhibit, is positively sloped, indicating that a change in the level of income or production induces a change in government purchases.

This new government purchases line actually indicates both induced government purchases and autonomous government purchases. The slope is induced government purchases and the intercept is autonomous government purchases.

• Slope: The slope of this new government purchases line is positive, but less than one. In this case the slope is equal to 0.05, a $1 change in aggregate income or production induces a $0.05 change in government purchases. This positive slope indicates induced government purchases. Moreover, the slope of the line is also the marginal propensity for government purchases (MPG).


• Intercept: This new government purchases line, like the original line, intersects the vertical axis at a positive value of $2 trillion. And once again this indicates autonomous government purchases.