Jeremy Grantham

Jeremy Grantham is a British-born investor, born in Ware, Hertfordshire in 1938. He read economics at the University of Sheffield and then took an MBA at Harvard Business School, remaining in the United States afterwards.

In 1969 he co-founded Batterymarch Financial Management in Boston, an early adopter of quantitative methods and of index-tracking products at a time when both were unusual. In 1977 he co-founded GMO with Richard Mayo and Eyk Van Otterloo, where he has served as chief investment strategist and later as long-term investment strategist.

His public reputation rests on the quarterly letters he has written for decades and on GMO's published long-horizon asset class forecasts. The framework behind both is mean reversion: he treats profit margins and valuation multiples as having long-run normal levels set by competition and by history, and treats prolonged departures from those levels as conditions that eventually correct. His research programme has been the identification of historical bubbles across markets and asset classes, and the argument that they have consistently deflated back toward trend.

He wrote publicly about the Japanese asset price bubble of the late 1980s, the technology stock bubble of the late 1990s and the United States housing bubble of the mid-2000s, and those letters are the documented basis of his reputation as a student of speculative episodes. He has also written extensively on resource constraints and climate change as economic issues, and in 1997 he and his wife Hannelore founded the Grantham Foundation for the Protection of the Environment, which has funded climate research institutes at the London School of Economics and Imperial College London.

Grantham's framework rests on the claim that the important financial series have long-run normal levels, and that competition is the mechanism enforcing them. Unusually high profit margins attract capital, capital increases capacity, and capacity competes margins back down. On that reading, a period of exceptional profitability is evidence about the future in the opposite direction from the way it is usually read, because the same forces that produced it are the ones that will erode it.

His bubble research applies the same logic to prices rather than to margins. The working definition he uses is statistical rather than intuitive: a departure from long-run trend large enough to be a genuine outlier rather than ordinary variation. The claim he draws from the historical record is that such departures have consistently ended by returning toward trend, though he has been consistent that this says nothing about when.

That last point is what separates the framework from a forecasting method, and he has been explicit about it. Valuation tells you what the long-run terms look like from here; it does not tell you what happens next year, and an extreme can become more extreme for a long time before it resolves. He has described being early as the recurring occupational cost of working this way.

The resource and climate work follows the same intellectual habit rather than being a departure from it. In both cases the argument is that a trend which cannot continue indefinitely will eventually stop, and that markets price such trends as though they will persist because the arrival date is unknown.

His quarterly letters made valuation-based, long-horizon analysis legible to readers who would never open an academic paper, and they are a common entry point into thinking about markets in terms of what conditions imply rather than what is likely to happen next.

The statistical definition of a bubble, as a measurable departure from a long-run trend, gave commentary a way to argue about the question with evidence rather than intuition, and that framing is now widely borrowed.