From one of my favorite professionals:
"... I think there's a lot of evidence that shows those very largest ones [are overvalued]. There's evidence that we've done on the S&P 500, where if you take the top market value stock every year over the 50 years of the existence of this index, there is tremendous underperformance of that strategy. Take the top 20 stocks, which is only 4 percent of the index in number, but one-third of the market value of the index, and it underperforms by more than 150 basis points per year over 50 years. So yes, those big-cap stocks do underperform and there is evidence that they do so significantly..."
Jeremy Seigel
A reason to consider Fundamental Indexing rather than Cap-weighted indexing.