Detroit’s bankruptcy filing is one depressing read. Poverty, crime, blight – you name the malady and there’s plenty of data to back it up. And unfortunately, Detroit’s not alone. You may be wondering which city hits the wall next.I’m not making predictions, but I’ve looked at one indicator that may offer some clues: population loss.As any good Ponzi Schemer will tell you, your future looks much better when there are more people moving in than moving out. Once the population change turns negative, a vicious circle can take hold, and that’s exactly what we saw in Detroit.