Whether or not you believe it would have made a difference to 'know' or not, the facts are that over the course of US economic history, you rarely know you're in a recession until long after it starts. Would you still chase day after day? Could you stand to watch the greater fools buying in the belief they are not the patsy? The following six facts might put things into perspective...
1) Think back to 2008, a couple of days before the Lehman failure. Looking at the data in hand, you would see GDP growth at about 1% in Q1 and 3% in Q2. More specifically, Q2 GDP growth had just been revised up on August 28 from 1.9% to 3.3%, sparking a 212-point Dow rally that day. http://www.nytimes.com/2008/08/29/business/29econ.html?_r=0
2) In March 2001, 95% of economists thought there would not be a recession, but one had already begun.
3) No economist predicted the 1990-91 recession beforehand.
4) Hardly any economists recognized the severe 1973-75 recession until almost a year after it started. Indeed, that recession began with the ISM at 68.1, and payroll jobs growth did not turn negative for eight months.
5) In 1970, unaware that the economy was nine months into recession, none other than Paul Samuelson said that the NBER had worked itself out of a job, meaning that improved policy expertise had made recessions very unlikely.
6) In three of the last 15 recessions - specifically, in 1980, 1945, and 1926-27 during the Roaring Twenties - stock prices remained in a cyclical upturn.