Interest

One important variable that would influence investment is the rate of interest, because interest is a key cost of investment.

If the investor must borrow to invest, clearly he or she must make enough to pay the "note," i. e. to repay the loan with interest.
Even if the investor takes the money from his or her bank account, the investor must give up the interest she would otherwise get -- and that's an "opportunity cost."

Here again we see the importance of opportunity cost reasoning in economics. Suppose, for example, that Jane Doe wants to open a gasoline service station. Instead of borrowing the money, she cashes out her money market account to pay the contractor who builds the station. She has given up the interest she would otherwise get from the money market account, and that is her opportunity cost of spending her own money to build the station.

Since interest is a key cost of investment, we would expect that higher interest rates would mean lower investment, and conversely.

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