BOSTON ( TheStreet) -- The death of Apple ( AAPL) co-founder and former Chief Executive Officer Steve Jobs on Wednesday may have jostled many investors into checking on how many Apple shares they actually own.Some will be surprised about how many they hold, given the tech firm's ubiquity as a top holding in mutual funds, pension funds and index funds. That recognition should prompt them to reassess their own portfolio's diversity, since a concentration in one stock, or one industry, may add to volatility and reduce returns. That's not to say Apple investors haven't been richly rewarded by their loyalty to the revolutionary maker of iPods, iPads and iPhones. The company's shares have a 10-year average annual return of a staggering 47% versus the S&P 500's 2.6% return. This year, Apple's shares are up 17% compared with the S&P 500's 7.6% decline.