The S&P 500 Index (SPX) has advanced an impressive 4.9% so far this year through Tuesday's open, but that gain is driven by a relative handful of stocks, according the Financial Times. Just 50 stocks, or 10% of the index, delivered 80% of that gain, per analysis by Fundstrat Global Advisors cited by the FT. In fact, if you take out the top ten gainers, the S&P would be up only about 2%, Fundstrat head of research Thomas Lee told clients in a note on Monday, per the FT. This lack of breadth in the market is a source of concern for some market observers.

The Top Ten

Technology giants Apple Inc. (AAPL), Facebook Inc. (FB) and Inc. (AMZN), all three with YTD gains of 20% or more through Monday, account for roughly 33% of the total S&P gain, the Wall Street Journal adds. Three other tech leaders round out the top ten: Google parent Alphabet Inc. (GOOG), Microsoft Corp. (MSFT) and Oracle Corp. (ORCL).

The remaining four stocks may be surprising to some: tobacco company Philip Morris International Inc. (PM), drug and health products company Johnson & Johnson (JNJ), consumer products maker Procter & Gamble Co. (PG), and payments facilitator Visa Inc. (V).

Relative to a number of historical benchmarks, the S&P 500 is seriously overvalued right now, per a report from Bank of America/Merrill Lynch analyzed by Investopedia. (For more, see also: S&P 500 Overvalued on Almost Every Metric.)

Effect of Market Cap

While the top ten have generated slightly over half the total advance in the S&P 500 so far this year, such concentration of gains is not that far out of line with historical experience. In a typical year, the ten leaders normally account for 45% of the market's total price movement, according to analysis by AQR Capital Management cited by the Journal.

A big factor is that the S&P 500 Index is weighted by market capitalization, and thus it heavily reflects moves by its biggest components in terms of market cap. Apple, for example, is the world's largest company by market cap, at $756 billion. Alphabet, Amazon, Facebook and Microsoft are all above $400 billion, and Oracle is at $186 billion. By contrast, the average market cap for the S&P 500 companies is $42 billion, per data from Siblis Research. (For more, see also: Why Mega Tech Stocks Will Win Longterm.)

Lofty Valuations

The price-earnings ratio (P/E) on the S&P 500 as a whole was 24 as of April 13, based on trailing 12 months' earnings as calculated by the Journal. The multiples on the ten stocks mentioned above are, in order from highest to lowest, per current Investopedia data: Amazon, 184, Facebook, 43, Visa, 36, Microsoft, 31, Aphabet, 30, P&G, 27, Philip Morris, 26, Oracle, 21, J&J, 21, and Apple, 17.

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