Billionaire investor Jeremy Grantham: This is the most expensive market in American history

• Grantham defines a bubble statistically as a 2-sigma event, noting all 26 historical instances broke back to their pre-existing trend.
• He states the current market is the 'most expensive in American history' and predicts a decline closer to 70% than 50%.
• Grantham compares the AI boom to past great inventions like railroads and the internet, which also saw overinvestment and temporary collapses where investors 'lost their shirts'.

Billionaire investor Jeremy Grantham asserts the current market is the 'most expensive in American history' and is in a bubble. He predicts a significant decline, potentially around 70%, and warns that the AI boom, while transformative, is following a classic bubble pattern of overinvestment that will lead to substantial investor losses.