By Leonid Bershidsky, Bloomberg View

Cheap oil is a double-edged sword for diversified economies: On one hand, investment drops in the oil and gas sector, hurting jobs and the stock market; on the other hand, consumers and companies that use a lot of hydrocarbons see their costs go down. In the U.S. and in Europe, the two effects will probably cancel each other out—as they have in the past.

In recent months, optimism about the effects of cheap oil has been widespread. Andrew Cunningham of Capital Economics estimated that a $10 fall in the price of oil transfers the equivalent of 0.5% of the world's gross domestic product from oil producers to oil consumers, who form the foundation of the U.S. and European economies. Charles Schwab's Liz Ann Sonders pointed out in November that consumer spending is 68% of U.S. GDP, while oil and gas capital investment represents only about 1%.

So everything's great for Western economies while Saudi Arabia and other oil producers pay for their bid to drive U.S. frackers out of business, right? Wrong, economists are beginning to realize, as they analyze historical data and cheap oil's effects on different industries.

Georg Zachmann, an economist at the Bruegel think tank in Brussels who looked at data on oil price and GDP changes for the years 1962-2014, estimates that a halving of the oil price is unlikely to add more than one percentage point to economic growth in the European Union. Economic research has shown that the negative effects of oil price rises are bigger than the positive effects of commensurate price drops.

A report released Jan. 20 by Moody's, the credit rating agency, provides an insight into why there's no cheap-oil bonanza in store for Western economies in 2015. One reason is that the oil sector—and consumers in regions that are dependent on it, such as Texas and North Dakota—is not the only one that lower prices will affect. There are also losses in steel pipe production, in construction materials for the fracking industry and in the sustainable energy business, which gets priced out of the market by cheap hydrocarbons. Airlines, shippers and producers of packaged foods benefit the most from lower oil prices—but their combined impact is not enough to tip the balance in favor of strong growth.

Here's Moody's map of how various industry sectors will be affected by cheaper oil:

It seems heavily weighted toward industries that should benefit, but Moody's predicts relatively small gains in most of those. The boost to consumer spending power, for example, is only expected to speed up U.S. retail sales growth to 4% to 5% this year from 3% to 4% in 2014. "We don't believe the increase in spending will be anywhere near proportional to the level of gas price reductions," Moody's wrote. In Europe, retailers may not see any gain at all because of the region's high fuel taxes.

Generally, Moody's predicts, the U.S. will benefit a little more than Europe—mostly because of the latter's taxation and regulation peculiarities. But then Europe, unlike the U.S., does not have a fracking boom, so its losses will also be proportionally less.

For developed economies, then, the net effects of cheap oil are close to zero—like those of moving furniture around an apartment. Production and investment will shift, as Zachmann describes it, "from energy-efficient to oil-intensive" sectors, from the oil industry to the rest of the economy. "But the aggregate impact may be smaller than suggested by the sectoral effects, the historical data and some media fanfare."

So what about the enormous amount of money—about $1 trillion by Zachmann's count—that should be transferred from oil-exporting countries to oil-importing ones this year? Most of that money will bypass the U.S. and Europe; Asia will get the windfall, and with it the challenge of capturing it successfully.

For the U.S., the challenge is different: It had better make sure the low oil prices do not kill its shale oil industry. If they do, and hydrocarbon prices start climbing, the rest of the U.S. economy will be hit by a shock disproportionate to this year's predicted modest gains, and there will be nothing to compensate for it. So far, the frackers are holding on. In the second week of January, they increased oil output again to 9.19 million barrels a day, but it's far from certain they will be able to withstand the current prices—$48 for a barrel of Brent oil today—much longer.