Monday's market, following a bullish Friday session that saw the S&P 500 Index (SNPINDEX:^GSPC) tack on 1.3%, was far more subdued, even after positive developments in the outlook for the U.S. economy. Ratings agency Standard & Poor's upgraded its outlook on the nation's credit rating from "negative" to "stable," just days after a Labor Department report detailed a steadily improving job market. While the S&P lost less than a point, ending at 1,642, three of its components managed to pile on losses.

Video game retailer GameStop (NYSE:GME) slumped 4.8% Monday, on the heels of Friday's massive 6.2% gain. Shareholders had bid up the stock after Microsoft announced used games would still function on the new Xbox One console. Still, the broader market's skepticism with the future of physical, used games remains visible: Last week, GameStop shares were the second-most shorted in the entire 500-stock index. 

IT and consulting services company Cognizant Technology Solutions (NASDAQ:CTSH) saw shares stumble 3.7% Monday. The stock, which had posted back to back gains around 2% before today's sell-off, is nearly 30% more volatile than the S&P, and shares have dipped below $65 after hitting highs above $80 in April. Despite its recent woes, the underlying business remains sound: Having posted four straight years of earnings-per-share growth above 20%, it's one of the fastest-growing IT outsourcing businesses in India.

Finally, American Tower (NYSE:AMT) slipped 3% Monday. As a REIT that operates thousands of towers enabling wireless communications, shares have amply rewarded longer-term investors in recent years as the demand for wireless devices has surged. But as American Tower's share price shot higher, its valuation got more and more expensive -- to the point where they currently fetch more than 50 times trailing earnings today.

Fool contributor John Divine has no position in any stocks mentioned. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

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