In a recent interview with Financial News, Research Affiliates partner Rob Arnott argued that the surge in valuations of the largest U.S. tech stocks is unlikely to continue, adding that the concentration of assets in a small number of companies is posing a threat to the bull market.

“Will these stocks produce such impressive growth that they will justify their current market cap, or are these implausible growth expectations? We do not have a crystal ball, of course, but we would recommend not betting on the momentum continuing,” said Arnott.

Arnott argues that Microsoft belongs in the category with the fast-growing tech stocks referred to as the Faang group. According to Arnott, the “Fanmag” group of stocks is “roughly 15% of the total $30 trillion market cap for all US stocks. This places these six companies above the market cap of all but two of 61 countries in the Morningstar Global Markets index.” He adds, “US healthcare is smaller than the Fanmags. That is startling.”

The article says, “The five Faang stocks declined from their highs, leading the market’s sharp sell-off, in late 2018, adding weight to calls by a number of commentators including analysts at Canaccord Genuity that the grouping constituted a bubble.”

According to Arnott, “marginal buyers do not care about valuation. They care about a story,” and argues that bitcoin’s 2019 recovery would suggest it “remains in a bubble: “If bitcoin truly becomes an accepted currency, no valuation model for it exists, other than what the public chooses to believe its worth,” adding, “Predicting near-term price direction is a fool’s errand, but any expectation of a higher price rests solely on the hope of selling to a future buyer at a higher price.”