U.S. companies have spent $1 trillion this year on buying back their own stock—a record figure reached three weeks before year's end. That's according to TrimTabs, an investment research firm.

One trillion dollars, a round number of epic proportions, is roughly equal to the amount U.S. consumers are expected to spend this holiday shopping season. It's also larger than the gross domestic product of 166 countries.

Just in the past week, pharmaceutical giants Pfizer and AbbVie announced a combined $15 billion in stock buybacks. Pioneer Natural Resources, an oil extraction company, announced a $2 billion buyback, and animal medicine maker Zoetis announced another $2 billion.

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"Stock buybacks have been going on throughout this bull market, and companies are using it partly as a way to prop up their prices," said Edward Yardeni, chief investment strategist at Yardeni Research. "Especially in this environment, they would like to do what they can to keep their prices from going down."

Buybacks boost a company's stock price by reducing the total number of shares it has in the market. With the three major U.S. stock indexes down on the year as of Monday, it's not clear buybacks have been much help. And the jury is out on how benficial to shareholders stock buybacks are long-term, with one 2016 analysis finding that companies that don't repurchase their own stock tend to perform better in the long term than those that do.

One implication of high buybacks is that companies don't have other high-return projects in which to put money, Cowan Asset Management wrote recently. "This means that, collectively, companies aren't investing in things like buildings and machinery – things that increase the overall economy's productivity."

For that reason, buybacks have drawn criticism not only from progressive economists but from such unlikely sources as Sen. Marco Rubio. The Florida Republican recently said on Twitter that companies who buy their stock are "deciding that returning capital to shareholders is better for business than investing in their products or workers. Tax code encourages this. No surprise we have work life that is unstable & low paying."

When corporation uses profits for stock buy back it’s deciding that returning capital to shareholders is better for business than investing in their products or workers. Tax code encourages this. No surprise we have work life that is unstable & low paying. https://t.co/yHV1qS0yLu — Marco Rubio (@marcorubio) December 13, 2018

The GOP-championed tax cuts that Rubio voted for last year have encouraged buybacks by making even more cash available to companies at at time when they are sitting on historically large cash reserves, without guidance from lawmakers on how to spend that money -- say, requirements to hire workers or boost pay in return for their tax windfalls, which the tax law did not include.

Shareholders often like buybacks, since they increase the value of the stock they already hold. But companies have historically had questionable judgment when it comes to timing their purchases, buying more of their shares precisely when they're highly priced.

Prior to this year, buybacks hit a high in the third quarter of 2007. One year later, the S&P 500 had lost one-third of its value.