Jeremy Grantham on AI, Bubbles and Why Mean Reversion Lives On

Jeremy Grantham joins Excess Returns to discuss The Making of a Permabear, mean reversion, market bubbles, AI, the Magnificent 7, and the long-term lessons investors can take from his career at GMO. We cover why he rejects the simple “permabear” label, how he thinks about valuation and bubbles, why AI may be both transformative and dangerous for investors, and why long-term thinking is so hard but so essential.
The Making of a Permabear: The Perils of Long-term Investing in a Short-term World
groveatlantic.com/book/the-making-of-a-permabear
GMO
gmo.com/americas
Grantham Foundation
granthamfoundation.org
Topics covered:
• Why Jeremy Grantham thinks the “permabear” label misses the point

• The difference between being generally bearish and making a true “abandon ship” call

• Mean reversion, valuation cycles, and why history still matters for investors

• Why monopoly power helped reshape U.S. profit margins and market concentration

• How AI could turn today’s monopoly winners into brutal competitors

• Why new technology often becomes a cost of doing business rather than a permanent profit boost

• How Grantham defines bubbles using two-sigma market events

• Lessons from Japan, the dot-com bubble, the housing bubble, and the 2021 speculative peak

• Why institutional investors struggle to stick with value strategies during bubbles

• The role of purpose, climate risk, toxicity, and long-term thinking in Grantham’s later career

• The one lesson Grantham would teach ordinary investors about pessimism, realism, and time horizons

Timestamps:
00:00 Jeremy Grantham on unpleasant news and long-term investing
04:18 Reinvesting when terrified in 2009
08:43 Why Grantham told investors to abandon ship in 2008
10:28 Mean reversion and why history matters
14:00 Monopoly power, the Mag 7, and rising market concentration
17:14 Why AI is important but impossible to forecast
20:21 AI as a cost of doing business
21:24 From monopoly profits to brutal AI competition
24:05 How investors should think about valuation mean reversion
27:00 Why high returns on capital should eventually attract competition
29:47 How Grantham defines a market bubble
33:00 Japan’s extreme bubble and GMO’s zero weight decision
34:19 The dot-com bubble and the pain of being early
38:00 Grantham’s bubble warning signal in 2021
41:35 Whether today’s market is showing classic bubble behavior
43:00 QuantumScape, meme stocks, and speculative excess
46:35 How ChatGPT interrupted the 2022 bear market
49:12 Investor behavior and the cost of underperforming in a bubble
55:00 Purpose, philanthropy, climate risk, and useful work
01:01:03 The one lesson Grantham would teach average investors