Writing in City Journal, Jim Manzi reminds us that the social sciences aren't real sciences. An excerpt:
But the situation was even worse: it was clear that we wouldn’t know which economists were right even after the fact. Suppose that on February 1, 2009, Famous Economist X had predicted: “In two years, unemployment will be about 8 percent if we pass the stimulus bill, but about 10 percent if we don’t.” What do you think would happen when 2011 rolled around and unemployment was still at 10 percent, despite the passage of the bill? It’s a safe bet that Professor X would say something like: “Yes, but other conditions deteriorated faster than anticipated, so if we hadn’t passed the stimulus bill, unemployment would have been more like 12 percent. So I was right: the bill reduced unemployment by about 2 percent.”
Another way of putting the problem is that we have no reliable way to measure counterfactuals—that is, to know what would have happened had we not executed some policy—because so many other factors influence the outcome. This seemingly narrow problem is central to our continuing inability to transform social sciences into actual sciences. Unlike physics or biology, the social sciences have not demonstrated the capacity to produce a substantial body of useful, nonobvious, and reliable predictive rules about what they study—that is, human social behavior, including the impact of proposed government programs.