Nancy Koehn, historian at the Harvard Business School, stopped by Boston Public Radio on Thursday to discuss the slowing growth of Silicon Valley tech companies, and the shifting interests toward profit and away from big spending.

“This represents a real reckoning,” Koehn said of the dwindling growth for companies like Uber and Lyft. “The 32 tech companies that went public this year thus far ... have only appreciated … on average, about 5%.”

Koehn drew comparisons between today’s tech industry and the railroad giants of the 1800s. “In all kinds of moments of hyper growth and technological change, investors in public markets have thrown their money at what look like high-growth opportunities,” she said. “Unicorn railroads that were trying to get to market on building the most track, the fastest, were the darlings of the then growing Wall Street in New York.”

"Well, guess what?" Koehn said. “A whole bunch of those companies went belly-up."

Koehn offered some optimism, however. “For most of us,” she said, “we’re holding our stock for retirement. We’re not holding it day trading.”

“For us, it’s really good news because it’s saying that the capital markets, in which the large bulk of the value of most of our retirement lies … are a hell of a lot better bet than the exuberance that we’re seeing — short-lived exuberance, of trying to find the next splashy winner that, ultimately, may be very vulnerable."

Nancy Koehn is a historian at the Harvard Business School where she holds the James E. Robison chair of business administration. Her latest book is "Forged in Crisis: The Power of Courageous Leadership in Turbulent Times."