Charles Schwab Chief Investment Strategist Liz Ann Sonders says that “traditional supports” remain in place to back the stock market’s recent gains, and also notes that some non-traditional supports are also present — for the moment, at least.

“There remain traditional supports for the market’s advance, notably very strong earnings and still-reasonable valuations,” Sonders writes in commentary on Schwab’s web site. “Compared to both the 2000 top and the 2007 top, earnings are notably higher, valuations more reasonable and interest rates (and inflation) much lower.”

But, Sonders says, despite the market’s rise, individual investors have remained lukewarm on equities. The gains, she said, are driven instead by a dwindling of supply (caused by near-record levels of stock buybacks by companies) and increased demand from some non-traditional sources. One is central banks. “Behind the heightened interest in stocks are growing central-bank reserves requiring increased diversification,” she writes. “In US dollar terms, the four largest central banks have expanded their balance sheets to more than $13 trillion, compared to only $3 trillion 10 years ago. Most central banks have had heavy and consistent reliance on fixed-income securities, but with yields low (and falling) in many countries, keeping all reserves in fixed income risks a declining value of reserves.”

In addition, Sonders says the number of bond mutual funds that own or are buying stocks is at its highest point in 18 years. “The trend is easy to understand when considering the quandary many fixed-income investors face,” she writes. “There’s been a bull market in bonds for more than 30 years, and yields, which move in the opposite direction of prices, are near record lows.”