Interest rates should be zero on average.
Interest rates go positive when there is too little saving and too much borrowing – banks have to bribe rich people to put their money in the bank so they can lend out that money to employed middle class people and sometimes poor people. A rapidly growing population of middle class working people living on stolen Native American Land is a great Ponzi scheme for positive interest rates and stock market booms.
Interest rates go negative when there is too much saving and too little borrowing – banks get lots of savings from the rich 1% but no good borrowers because the 99% poor deflated sheeple are living on welfare, benefits, junk food, Obamacare, homeless, in prison, in rehab, in college, in the hospital, dying, or dead. So the rich put their money into a bubble commodity or stock market and hope they can get most of that money back, but they may lose 50% or more. A negative 10% interest rate on bank savings accounts is maybe the best they can hope for. All prices collapsing, deflating or teetering on the edge of deflation, revolution or worse. Not much of a stable domestic middle class any more. Globalization evaporation blur into teeming impoverished deflated masses, illegals, Muslims, beheadings drug dealers, addicts, overdoses, wars, ….
Equilibrium is when a zero interest rate balances the dollars of saved and dollar borrowed.
Often they ignore that balance depends on theft and continued occupation of Native American land and resources.
Many classes on such topics at every university that are key to understanding the issues.
Lots of literature on these topics sometimes using keywords “Golden Rule”