The stock pickers among you have fewer than half as many stocks to choose from as you did 20 years ago. Perhaps the most revealing statistic in this regard is the percentage of corporate profits that come from the 100 most profitable listed firms. In 1975 that percentage was 48.5%, according to research conducted by finance professors Kathleen Kahle of the University of Arizona and Rene Stulz of Ohio State. Twenty years later, in 1995, that percentage had grown only slightly to 52.8%.