The Big Boppers

It's been a strong year so far for the S&P 500 index, rising nearly 8 percent on the year. But it's been remarkably top-heavy: If we strip out the performance contribution of the five largest stocks in the S&P 500, the index is up only 4.6 percent.

Those five biggest stocks are:

• Apple,  up 16 percent
• Facebook, up 32 percent
• Amazon, up 33 percent
• Microsoft, up 13 percent
• Alphabet,  up 25 percent

At the start of the year, these five stocks accounted for 11.6 percent of the index’s market cap. That share stands at about 13.7 percent today.