Here's the introduction to a paper I'm giving at the AEA meetings (journal version of paper). The model in the paper, which is a variation of the Brock and Hommes (1998) generalization of the Lucas (1978) asset pricing model, shows that bad advice from experts can increase the likelihood of harmful financial bubbles:
Bad Advice from Experts, Herding, and Bubbles: The belief that housing prices would continue to rise into the foreseeable future was an important factor in creating the housing price bubble. But why did people believe this? Why did they become convinced, as they always do prior to a bubble, that this time was different? One reason is bad advice from academic and industry experts. Many people turned to these experts when housing prices were inflating and asked if we were in a bubble. The answer in far too many cases – almost all when they had an opinion at all – was that no, this wasn’t a bubble. Potential homebuyers were told there were real factors such as increased immigration, zoning laws, resource constraints in an increasingly globalized economy, and so on that would continue to drive up housing prices.
When the few economists who did understand that housing prices were far above their historical trends pointed out that a typical bubble pattern had emerged – both Robert Shiller and Dean Baker come to mind – they were mostly ignored. Thus, both academic and industry economists helped to convince people that the increase in prices was permanent, and that they ought to get in on the housing boom as soon as possible.
But why did so few economists warn about the bubble? And more importantly for the model presented in this paper, why did so many economists validate what turned out to be destructive trend-chasing behavior among investors?
One reason is that economists have become far too disconnected from the lessons of history. As courses in economic history have faded from graduate programs in recent decades, economists have become much less aware of the long history of bubbles. This has caused a diminished ability to recognize the housing bubble as it was inflating. And worse, the small amount of recent experience we have with bubbles has led to complacency. We were able to escape, for example, the stock bubble crash of 2001 without too much trouble. And other problems such as the Asian financial crisis did not cause anything close to the troubles we had after the housing bubble collapsed, or the troubles other bubbles have caused throughout history.
Economists did not have the historical perspective they needed, and there was confidence that even if a bubble did appear policymakers would be able to clean it up without too much damage. As Robert Lucas said in his 2003 presidential address to the American Economic Association, the “central problem of depression-prevention has been solved.” We no longer needed to worry about big financial meltdowns of the type that caused so many problems in the 1800s and early 1900s. But in reality economists hardly knew what to look for, did not fully understand the dangers, and were hence unconcerned even if they did suspect that housing prices were out of line with the underlying fundamentals.
A second factor is the lack of deep institutional knowledge of the markets academic economists study. Theoretical models are idealized, pared down versions of reality intended to capture the fundamental issues relative to the question at hand. Because of their mathematical complexity, macro models in particular are highly idealized and only capture a few real world features such as sticky prices and wages. Economists who were intimately familiar with these highly stylized models assumed they were just as familiar with the markets the models were intended to represent. But the models were not up to the task at hand, and when the models failed to signal that a bubble was coming there was no deep institutional knowledge to rely upon. There was nothing to give the people using these models a hint that they were not capturing important features of real world markets.
These two disconnects – from history and from the finer details of markets – made it much more likely that economists would certify that this time was different, that fundamentals such as population growth, immigration, financial innovation, could explain the run-up in housing prices.
The model in this paper examines the implications of these two disconnects and shows that when experts endorse the idea that this time is different and cause herding toward incorrect beliefs about the future, it increases the likelihood that a large, devastating bubble will occur.
 See Wieland and Wolters (2011) for an overview of the forecasting performance of macroeconomic models before, during, and after the crisis.