> When the economy is growing, investment makes sense. Why put your money under the mattress when it could be out there, working for you?

> When the reverse happens, investment stops making sense. Why risk your money when it becomes worth more while it is sitting under your mattress?

Is it necessary to have a growing population in order to have a growing economy?

For much of human history I'd guess the answer was yes, because the size of the economy was based almost entirely on how much physical work people did, but the modern economy is very different from historical economies.