Economist Steve Hanke's bubble detector is signaling that the US stock market has entered bubble territory. The bond-stock yield spread, a secondary indicator, confirms the warning. The data behind the alarm is stark: US equities added roughly $10 trillion in market value over just 39 days, with the Nasdaq climbing 27% since March 30 to hit 29,000, and the S&P 500 rising 17% to 7,400.
Bond-Stock Yield Spread Confirms the Signal
Hanke's bubble detector relies on a combination of metrics. The bond-stock yield spread — the difference between long-term Treasury yields and stock earnings yields — has tightened to levels historically associated with overvaluation. While Hanke hasn't publicly detailed the exact threshold, the dual confirmation from both the primary detector and the spread adds weight to the warning. The last time this combination triggered was before the 2000 dot-com crash.
Mega-Cap Tech Drives the Rally
Five mega-cap tech stocks — Alphabet, Nvidia, Amazon, Broadcom, and Apple — have accounted for roughly half of the S&P 500's 12% gain since April 1. Alphabet led the pack with a 38% surge, while the others posted gains between 21% and 33%. That concentration has skewed the headline index. Over the same period, the equal-weighted S&P 500, which gives each company the same share, rose just 6%. The gap suggests the rally is narrow and driven by a handful of names, a pattern that often precedes sharper corrections.
Record Options Volume and Retail Inflows
Traders have piled into call options at an unprecedented pace. S&P 500 call option notional volume hit a record $2.6 trillion, representing 58% of all options traded — the highest share on record. That level of speculative activity typically accompanies late-cycle exuberance. Retail investors, meanwhile, poured $1.1 billion into tech hardware stocks during the week ending May 6, marking the second-largest weekly inflow on record and the fifth straight week of net buying. The combination of heavy call option activity and retail chasing tech stocks echoes patterns seen in previous market tops.
Concentration Raises Questions
The rally's narrow base leaves the broader market vulnerable. If the mega-cap tech stocks stall or reverse, the S&P 500 could face a disproportionate hit. The equal-weighted index's tepid 6% gain suggests most stocks aren't participating in the euphoria. Hanke's detector doesn't predict timing, but the historical track record of his tool has drawn attention from investors who remember the aftermath of the 2000 bubble. The next key data point will be whether the bond-stock yield spread widens again — a move that would signal the bubble is starting to deflate.
