UBS has said US investors' fear of missing out on a bull market rally has been the key recent driver of market gains, particularly for artificial intelligence stocks.
The wealth management arm of the Swiss bank pointed to clear signs of that anxiety among retail investors.
It noted that a leveraged exchange-traded fund linked to SK Hynix, a Korean chipmaker, had become the largest fund tied to a single stock, holding more than $10 billion.
Investors are also buying dips quickly, with the S&P 500 taking only three days to recover almost fully from a 4.5% fall early in June.
Jason Draho, UBS's head of asset allocation for the Americas, argued that the economic backdrop should keep the bull market running.
Most estimates put US growth at an annualised 2.5% to 3% in the second quarter.
A national average petrol price below $4 a gallon, down 14% over the past month, suggested inflation had probably peaked.
Draho described the regime as reflationary, a positive for risk assets, and more likely to soften into steady, low-inflation growth than to stagnate.
He played down concerns about the Federal Reserve after a hawkish policy meeting, pointing out that officials still only expect one interest rate rise this year, though this is a shift from March, when none was forecast.
But Kevin Warsh, the new Fed chair, stressed price stability and downplayed the rate projections as a guide to action.
Draho said the US equity rally ultimately depends on AI meeting the high expectations priced into markets.
"Investor positioning is elevated but not extreme, and generally more indicative of a positive outlook for risk-taking than sentiment indicators would suggest," he said.
"The continuation of the bull market ultimately hinges on AI delivering on the expectations currently built into market prices, but the macro regime is doing its part and that's likely to last for a while."
