Final answer:
An expansionary fiscal policy can decrease planned investment if it causes higher interest rates, discouraging borrowing and spending.
Step-by-step explanation:
An expansionary fiscal policy, with tax cuts or spending increases, is intended to increase aggregate demand. However, if it also causes higher interest rates, it can discourage firms and households from borrowing and spending, which reduces aggregate demand. This phenomenon is known as crowding out, where government borrowing and spending result in higher interest rates, leading to decreased business investment and household consumption.