It’s important for the American economy to have start-up companies — and not just so that the owners of those companies can earn their way into the middle class. Start-ups also help oil the engine of capitalism.

Ideally, there’s a sort of natural selection constantly happening in free markets. Weaker, less adaptable firms shrink and fail. Stronger, more innovative ones grow and thrive. Economists call that process “dynamism” or “creative destruction.” Its purpose is to make sure the country is employing people, money and technology in the most efficient way. When someone starts a company that tries to innovate — to sell a better, faster, cheaper way to solve a human problem — economists call that “productive entre­pre­neur­ship” because it helps the whole economy work better.

Today’s economy, though, has lost some of the dynamism it had after World War II, when shared prosperity flourished. Fewer new firms are springing to life, and fewer old firms are failing.

In a recent paper for the Brookings Institution, economists Robert Litan and Ian Hathaway calculate that a third of all U.S. firms were at least 16 years old in 2011. That’s a 50 percent increase from 1992. Nearly three-quarters of all private-sector workers were employed by those firms, up from three-fifths in 1992.

This brings us to the second problem with U.S. entre­pre­neur­ship today: Those older firms appear to be growing more interested in what economist William Baumol called “unproductive entre­pre­neur­ship.”

Put simply, that means companies are ramping up their efforts to win favors from the government — tax breaks, spending contracts or industry regulations that favor their firm over potential competitors. Many economists, such as Luigi Zingales of the University of Chicago, contend those efforts divert resources that could be boosting the economy and sparking more job creation.

From 1998 to the peak of the influence boom in 2010, after adjusting for inflation, American companies nearly doubled the money they spent lobbying federal lawmakers, according to the nonprofit Sunlight Foundation. There’s an index that tracks stock performance of the 50 companies that lobby the most, and in 2012, it outperformed the market as a whole by 30 percent.

A recent study for George Mason University’s Mercatus Center by economists Russell Sobel and Rachel Graefe-Anderson found that for companies, deep political connections (including high lobbying spending) and higher revenues go together. But instead of banking those extra revenues as profits, the firms appear to pass them on to their chief executives. The paper finds “a robust and significant positive relationship between political activity and executive compensation.”