Bank of America puts the peak concentration levels of several major historic bubbles side by side: the railroad era at 63%, Japan’s stock market at 44%, the internet bubble at 41%—and today AI-weighted “Big 10” stocks are also up to 41%.

Several times when the market topped, they all hovered around 40%. High concentration doesn’t automatically mean a top, but if it can climb to this line, it has never been cheap. How to position your holdings is up to you to weigh.