Wait until next month, economists said. That's when consumers will spend their bonus savings from lower gas prices.

Next month (January) came and went with the same disappointing results as December: a big decline in U.S. retail sales. A 9.3 percent plunge in gas station sales, driven by falling prices, was the main, but hardly the only, culprit behind January's 0.8 percent decline. (Retail sales are reported in nominal terms). Excluding gas station sales, retail sales were unchanged in January after a 0.2 percent decline in December.

Retail "control," a category that excludes sales of gas, food, autos and building materials, and serves as a direct input for consumer spending in the GDP report, rose 0.1 percent last month following a 0.3 percent December decline. Consumer spending, two-thirds of which is on services, is a lot stronger than retail (goods) sales. And unlike goods prices, services prices aren't falling.

For months, economists have been touting lower oil prices as an unqualified plus for the economy, especially consumers. They seemed less concerned with the driver of the 59 percent decline in U.S. benchmark crude prices between June and January: changes in supply or demand. In economists' macro world, prices often exist in isolation.

Of course, in the micro universe, lower prices may be a reaction to increased supply, a reflection of weaker demand, or a combination of the two--with very different implications for the quantity sold, which is what really matters.

The conventional wisdom holds that oil prices collapsed because of a positive supply shock. As technological innovation enabled energy companies to extract previously untapped shale oil and gas at increasingly lower costs, producers were willing to supply more at any given price than they were before. And yes, that is an unqualified plus.

That storyline has some glaring gaps. For example, domestic crude oil production has been steadily increasing for six years, practically doubling in that time period to a record 9.2 million barrels a day, according to the U.S. Department of Energy. Various organizations report global oil supply on a weekly basis, so the supply glut should not have come as a huge surprise to anyone.

What changed was the outlook for global demand. Starting in late September, a combination of disappointing economic news from Europe, China and other developing nations and dashed hopes of more aggressive action by the European Central Bank to stem deflation sent crude oil prices into freefall. (The ECB finally announced a an open-ended program of quantitative easing in January.)

My suspicion is the decline in crude oil prices reflects a gradual supply shock compounded by a sudden, expected demand shock as prospects for global growth were ratcheted down. Speculators and hedgers accelerated the sell-off, sending crude prices tumbling as much as 4 percent on some days. Prices have stabilized for the moment as the market seeks a new equilibrium.

If it turns o ut that reduced demand for crude oil is the primary culprit - if lower prices are an effect, not a cause - then the expected bonus from lower oil prices may not materialize.

This reaction to oil prices is hardly unique to the present circumstances. For some reason - toxic fumes? - when it comes to oil, economists tend to forget their basic training. If you were to carry the running commentary to its logical conclusion, you would think that lower oil prices increase economy-wide demand and higher prices reduce it. This is what I call loop-de-loop economics because there is no way out. To think we spend so much time obsessing over monetary and fiscal policy when we could simply cede demand management of the U.S. economy to some Middle East potentate!

Economists have been grudgingly forced to acknowledge the effects of lower oil prices on producers as energy companies report lower profits, cut back on their drilling activity, reduce staff and table plans for new exploration and development. Those effects - lower prices, output and profits - are indicative of weak demand.

Whatever is affecting crude prices seems to be taking its toll on other industrial commodities. The BLS Raw Industrials Index is down 14 percent in the last 10 months. Unless producers of raw materials, such as copper and steel scrap, zinc, tin and cotton, have had their own fracking-type revolution that reduces costs, one is tempted to conclude that weaker global demand is responsible.

If retail sales bounce back this month, economists will point to the data as proof that consumers are spending their oil-price tax cut, or whatever they like to call it, after all. As for me, I'm dusting off the script so I'll be ready for the next spike in oil prices.

Caroline Baum is a contributor to e21. You can follow her on Twitter here.