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It’s hard to overstate the excitement of tech people about what is on the verge of happening to the practice of medicine.

Eric Horvitz, co-director of Microsoft Research’s main lab in Redmond, Wash., told me about a system that could predict a pregnant woman’s odds of suffering postpartum depression with uncanny accuracy by looking at her posts on Twitter, measuring signs like how many times she used words like “I” and “me.”

Ramesh Rao of the California Institute for Telecommunications and Information Technology at the University of California, San Diego, described how doctors using video and audio to remotely assess victims of stroke made the correct call 98 percent of the time.

This is just the beginning. “The real innovative things have yet to be activated,” Rao said. “Whatever happens will be disruptive.”

That’s not the half of it.

A few years ago, this kind of technological development would be treated like unadulterated good news: an opportunity to improve the nation’s health and standard of living while perhaps even reducing health care costs and achieving a leap in productivity that would cement the United States’ pre-eminent position on the frontier of technology.

But a growing pessimism has crept into our understanding of the impact of such innovations. It’s an old fear, widely held since the time of Ned Ludd, who destroyed two mechanical knitting machines in 19th century England and introduced the Luddite movement, humankind’s first organized protest against technological change.

In its current incarnation, though, the fear is actually very new. It strikes against bedrock propositions developed over more than half a century of economic scholarship. It can be articulated succinctly: What if technology has become a substitute for labor, rather than its complement?

As J. Bradford Delong, a professor of economics at the University of California, Berkeley, wrote recently, throughout most of human history every new machine that took the job once performed by a person’s hands and muscles increased the demand for complementary human skills – like those performed by eyes, ears or brains.

But, Delong pointed out, no law of nature ensures this will always be the case. Some jobs – nannies, say, or waiting tables – may always require lots of people. But as information technology creeps into occupations that have historically relied mostly on brainpower, it threatens to leave many fewer good jobs for people to do.

These sorts of ideas still strike most mainstream economists as heretical, an uncalled-for departure from a canon that states that capital – from land and lathes to computers and cyclotrons – is complementary to labor.

It was a canon written by economists like Robert Solow, who won the Nobel in economic science for his work on how labor, capital and technological progress contribute to economic growth. He proposed more than 50 years ago that the share of an economy’s rewards accruing to labor and capital would be roughly stable over the long term.

But evidence is emerging that this long-held tenet is no longer valid. In the United States, the share of national income that goes to workers – in wages and benefits – has been falling for almost half a century.

Today it’s at its lowest level since the 1950s while the returns to capital have soared. Corporate profits take the largest share of national income since the government started measuring the statistic in the 1920s.

This shift is happening globally. In a recent article in the Quarterly Journal of Economics, Loukas Karabarbounis and Brent Neiman from the University of Chicago’s Booth School of Business found that the share of income going to workers has been declining around the world.

As the cost of capital investments has fallen relative to the cost of labor, businesses have rushed to replace workers with technology.

“From the mid-1970s onwards, there is evidence that capital and labor are more substitutable” than what standard economic models would suggest, Neiman told me. “This is happening all over the place. It is a major global trend.”

The implication is potentially dire: The vast disparities in the distribution of income that have been widening inexorably since the 1980s will widen further.

This is hardly a consensus reading of the record. “It is hard to make a very definite prediction about how the capital-income share will evolve over the next 10 years,” Daron Acemoglu, a colleague of Solow’s at MIT, told me. “Future technology could maybe increase the contribution of labor.”

Tyler Cowen, a professor of economics at George Mason University, argues that the very definitions of labor and capital are arbitrary. Instead, he looks around the world to find the relatively scarce factors of production and finds two: natural resources, which are dwindling, and good ideas, which can reach larger markets than ever before.

If you possess one of those, then you will reap most of the rewards of growth. If you don’t, you will not.

Conventional wisdom in economics has long held that technological change affects income inequality by increasing the rewards to skill – through a dynamic called “skill-biased technical change.” Losers are workers whose job can be replaced by machines (textile workers, for example). Those whose skills are enhanced by machines (think Wall Street traders using ultrafast computers) win.

It is becoming increasingly apparent, however, that this is not the whole story and that the skills-heavy narrative of inequality is not as straightforward as economists once believed. The persistent decline in the labor share of income suggests another dynamic. Call it “capital-biased technical change” – which encourages replacing decently paid workers with a machine, regardless of their skill.

For instance, research by the Canadian economists Paul Beaudry, David Green and Benjamin Sand finds that demand for highly skilled workers in the United States peaked around 2000 and then fell, even as their supply continued to grow. This pushed the highly educated down the ladder of skills in search of jobs, pushing less-educated workers further down.

This dynamic opens a new avenue for inequality to widen: the rise in the rewards to inherited wealth, a topic explored in depth in Thomas Piketty’s expansive new book, “Capital in the Twenty-First Century.”

The only safe route into the future seems to be to already have a lot of money.