There are some mind blowing graphs such as S&P Correlation vs Volatility
An interesting excerpt
"We believe that High-Frequency Trading activity has increased correlations, reduced volatility, and increased the intraday tail risk. In order to understand how HFT activity can impact the market, we will look at two common HFT strategies: index arbitrage and optimal execution of orders. Index arbitrage is an example of HFT arbitrage trading. As shown in Figure 5, current index volumes are significantly larger than total cash volumes, and a good amount of index derivative volume (Futures, ETFs) will not be directly offset by trades in cash securities. If the index price diverges from the prices of underlying constituents, index arbitrage HFT will act to realign them. For instance, if a group of stocks outperforms the index, an arbitrage program may sell these stocks and buy the index, causing their prices to realign. This trading activity will dampen the volatility of stocks and increase their correlation to the rest of the stocks in the index. HFT index arbitrage also facilitates the transfer of the market impact on futures and ETFs to the underlying stocks, thus providing a link between the high percentage of index trading and correlation of individual stocks. Another HFT trading strategy is a statistical arbitrage. A simplified example is a pair trade between two correlated stocks. If the price of one stock increases relative to the other, an arbitrage program will sell the outperforming stock and buy the underperforming one, thus reducing the volatility of both and increasing the correlation between the two."
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