The biblical proscription against usury goes all the way back to Moses. The rule was simple: you couldn't charge interest on a loan to anyone in your tribe. Strangers, on the other hand, paid interest. This isn't a matter of ancient biblical archeology; the edict against interest stuck for thousands of years, until around the time of Columbus.

It's worth taking a minute to understand the reasoning here. If money circulates freely within the tribe, the tribe will grow prosperous more quickly. I give you some money to buy seeds, your farm flourishes, and now we both have money to give to someone else to invest. The faster the money circulates, the better the tribe does. The alternative is a tribe of hoarders, with most people struggling to find enough resources to improve productivity.

Obviously, there's another force at work here. When I make an interest-free loan to you, I'm trusting you and giving you a gift at the same time. This interaction increases the quality of our bond and strengthens the community.

Seth Godin