# Why do we want MSB = MSC not MSB > MSC?

Why do economists want Marginal Social Benefit to equal Marginal Social Cost not be greater than?

Suppose, at the status quo, the MSB is 5 and the MSC is 3 (so MSB > MSC). That means if we increased the activity by one unit we would get 5 units of extra benefit and 3 units of extra cost, so the net change in welfare is $5-3=+2$. Thus, the status quo can't be optimal because we could increase welfare by adding an extra unit of the activity.
Suppose, at the status quo, the MSB is 3 and the MSC is 5 (so MSB < MSC). That means if we reduce the activity by one unit we would loose 3 units of benefit but save 5 units of cost, so the net change in welfare is $-3+5=+2$. Thus, the status quo can't be optimal because we could increase welfare by reducing the activity by one unit.