Can a mathematically sound prediction interval have a negative lower bound?

Is there a sound economic theory which predicts an 80 year cycle of great depressions?

• In a book by Arthur C. Clarke (Ghost from the Grand Banks if I remember correctly... if I'm wrong please correct me) he talks about an economic theory which predicts great depressions every 80 years or so. Now, current events have brought this prediction rather starkly to mind. Was there a sound basis for that prediction or is it merely that bad economic shocks happen often enough for a fake pattern to emerge? Was it maybe just a lucky guess? I've done a lot of googling of 80 year cycle and other terms that I've thought might get me somewhere interesting but all I get is conspiracy theorists and worse. Now, I know that Ghost from the Grand Banks is lesser Clarke but I'd like to think that he was a little bit more up on his economics than internet conspiracy theorists. The 80 year cycle thing might also have been mentioned in an introduction or afterword.

• Answer:

  Maybe some variation on the Kondratieff wave? Clarke's fictionalised spin on it? In any case, people generally use Kondratieff as a catch-all term for all hypothesised long waves in the economy. But they are in no way a sound economic theory, either theoretically or empirically. No one has any plausible reason for why these cycles would exist, though many 'Just-So' stories are proposed. Similarly, the evidence is not compelling. Given so much data, it would be surprising if you couldn't find some patterns if you looked hard.