Wealthion · Tuesday, September 1, 2026
David Rosenberg argues that market bubbles are driven by investor behavior and leverage, not solely by valuations. He points to soaring margin debt, high bullish sentiment, and portfolio managers being nearly fully invested as indicators of herd mentality and potential excess.
“The bubble is in investor behavior.”
“Why why did Charles Mai write that book on, you know, on the madness of crowds >> and and and and why did Bob Ferrell in the 1950s invoke sentiment into his technical work, the herd mentality?”
“It's not just about valuations. It's about what is the what is the behavior?”