The United States has experienced economic downturns, or recessions, roughly every 5-10 years since they began being tracked in the late 18th century. 1785, 1789, 1796, 1802, 1807, 1812, 1815, 1822, all the way to 1937, 1945, 1949, 1953, 1958, 1960, 1969, 1973, 1980, 1981, 1990, 2000, and most recently 2007.
What I'm confused about is, assuming the economy falls on some balanced distribution around the mean, there should be equally as many economic upturns, booms, or manias if we want to borrow a term from psychology, as there are downturns, recessions, and depressions.
Yet as far as I can tell, this isn't the case. The economy goes from neutral to negative and back, with an occasional foray into positive, showing a clear bias for negative periods over positive ones.
Why is this?