...I’d also offer a geographic hypothesis. For years, much of the growth in the nation revolved around the movement of people and business from the Rustbelt to the Sunbelt. Before long, a self-reinforcing dynamic took hold where growth begot more growth. As big corporations moved work from Pennsylvania and Ohio, entrepreneurs saw the opportunity to provide the new factories with everything from parts to catering services. Small contractors sprang up to build houses for the employees of all those new ventures, followed by other firms to provide restaurant meals and yoga instruction to all the new inhabitants. Only when the bubble burst did it become clear that all that growth-induced growth had gotten way ahead of itself. The entrepreneurial activity, along with the explosive job growth, came to a sudden halt....