Apple stock's influence on markets grows

— -- The huge growth in Apple's stock price that made it the USA's most-valuable company has also made it Wall Street's $600-a-share gorilla in the room.

Investors are finding gyrations in this one stock can have an outsized influence on the market. And with the stock resuming its upward trajectory, investors hope the Apple effect will help put a shine back on the market.

Shares of Apple soared $49.72 to $610 Wednesday after the consumer electronics maker's positive first-quarter earnings report late Tuesday. That move single-handedly contributed more than 5.13 points to the Standard & Poor's 19-point gain to 1391.

"Apple is absolutely critical from an investor psychology standpoint," says Frank Longman, director of trading strategies at Brean Murray Carret.

Apple's large and growing influence on the stock market is apparent from its:

•Weighting in broad market indexes. Apple accounts for more than 4% of the broad Standard & Poor's 500 index and is expected to account for 5.4% of its first-quarter profit. Without Apple, the S&P 500 this year would be up 9.2%, not 10.6%.

•Domination of tech-heavy indexes. Apple is a giant contributor to the value of tech indexes. It's 18% of the Nasdaq 100 index, for instance, which contains the 100 largest non-financial stocks in the Nasdaq composite. And it's nearly 12% of the Nasdaq composite, giving Apple twice the weighting of No. 2 Microsoft.

•Financial resources. Apple has $110 billion in cash and investments, making it one of the most flush companies in the S&P 500.

Its huge pile of cash, and its announced $9.9 billion dividend, makes Apple the third-largest dividend payer behind ExxonMobil and AT&T.

Despite Apple's oversized importance, though, it's still not hijacking the stock market, says Dan Greenhaus, strategist at BTIG. Even at a 4% weighting in the S&P 500, it's still below IBM's record 6.3% slice in December 1982.

Even if Apple shares went to $0 in a single session, and all other 499 stocks in the index were flat, the S&P 500 would only have its worst day since August 2011, says S&P's Howard Silverblatt.

And it's not just theoretical. Even while Apple shares had been down 13% from their 52-week high earlier this week, the market was still holding in, Greenhaus says.

Eventually, the scores of investors who own Apple stock may look to sell and lock in gains. But so far, they are hanging on.

"The big question is when investors will take profits," says Silverblatt says. "But so far, that hasn't happened."