Bloomberg | US Stock Market Concentration Is at Highest in Decades

BBG:The S&P 500’s 10 biggest stocks now account for roughly 40% of the index, a level of concentration not seen since the 1960s.

That has put some investors off, including Wall Street research veteran Ed Yardeni, who said in December that it no longer makes sense to recommend overweighting tech stocks.

Market historians argue that, while the concentration seems extreme relative to recent memory, there are precedents.

Top stocks as a share of the US market were at similar levels in the 1930s and 1960s, according to London Business School professor Paul Marsh, who studied the past 125 years of global asset returns.