Complex sentence: Historically, short selling is going against the upward trend of the stock market, with the S&P 500 and S&P 90 index realizing an average gain of approximately 9.77% return between 1926 and 2011.
Split into simpler sentences: Historically, short selling is going against the upward trend of the stock market. In the USA, the S&P 500 index has realized an average gain of approximately 9.77% return between 1926 and 2011.