I said in an entry last week that a downgrade from Morgan Stanley was a reasonable candidate for the cause of a downward stock price move, as we intuitively understand the idea of cause. But proving this would be a trickier matter. It would require showing that there was nothing else happening that evening or morning that may also have had consequences for that demand. Or, if there were other things happening, if would require some measure by which we could distinguish this causation candidate as more potent that the others. Even if it is a very general rule that similar announcement proceeds stock price fall, other explanations are possible. After all, since we’re assuming that Morgan Stanley’s analyst was working from publicly available information, we could hypothesize that a lot of traders and in-house buy-side analysts reached the same conclusion at the same time Joe Smith did and would have reached it even if Joe Smith had had nothing to say, or had through some analy...