[prottog]

> Interest rates have been declining for 400 years

This is surprisingly easy to explain: interest rates are a combination of the time factor of money (a dollar in hand is more useful than a dollar tomorrow) and the risk of default. Geopolitical stability and security has increased in the aggregate over the past few centuries, so it follows that interest rates have declined in aggregate.

Makes you wonder what the implications are, now that we've hit zero. ;-)