By Steven Russolillo

Updated March 16, 2016 8:52 p.m. ET

The corporate buyback binge continues at full tilt. But, perhaps with good reason, investors may be losing faith in its ability to propel share prices.

U.S. companies authorized $158 billion of new stock buyback programs in January and February, according to Birinyi Associates Inc. While there is no guarantee the rest of 2016 will maintain such a pace, it marked the best start to a year since the research firm started tracking this data in 1984.

February, in particular, stood out, as cash-rich companies said they would continue rewarding shareholders rather than investing. It was the fourth strongest month on record. Cisco Systems Inc. and Gilead Sciences Inc. led the pack with the heftiest announcements.

Even so, the S&P 500 Buyback Index, which contains stocks with the highest ratio of buybacks to market value, has fallen by 7% over the past year. That compares with a slight gain for the S&P 500 on a total-return basis.

The explanation may be that investors know how buyback booms end: When the market loses momentum, companies tend to get skittish about spending money on their own shares, even though they are on sale.

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For example, in booming 2007 when the last bull market peaked, buybacks totaled $761 billion. That plunged to $387 billion in 2008 and just $149 billion in 2009, according to Birinyi.

Based on what companies have announced, and also some evidence of what they have spent, such a pullback hasnít happened this time. But the fact that a company says it will repurchase stock doesnít mean it will. In 2015, companies bought back about four-fifths of the authorized amount.

So, one explanation for the sputtering Buyback Index is that investors smell trouble. Another explanation is more benign: That investors are optimistic about the economy and are rewarding companies that plow money into productive assets.

After all, if a company can get a higher return on a dollar than one returned to investors, it should retain and spend it. Capital expenditures, research and development and even wage increases have largely taken a back seat in this bull market.

The buyback express isnít slowing. But more investors want to switch trains.

ó tape@wsj.com