The new year has gotten off to a lousy start, at least on Wall Street, leaving us to wonder what might have changed. The stock market has been in a bullish warp for nearly six years, after all, blithely oblivious to all manifestations of reality save central bank easing. Lately, however, sellers seem to be dictating the terms.  Are investors, in their supposed collective wisdom, looking ahead and seeing recession or worse on the economic horizon?  We’ve never believed them capable of such prescience. Much more appealing is the idea that coldly mechanical market cycles color our perceptions of  the economy — so much so that a steep downturn in stocks can trigger recession, rather than the other way around. Whatever the case, the unaccustomed weakness of the U.S. stock market so far this year is troubling, since damned near everything that matters is riding on the continued, upward drift of share prices.