S&P correlates to several indicators which are interconnected.
1. US economic index since the companies are American
2. The USEI is partially driven by Fed (FOMC)
3. The S&P correlates to USD power and USD/Yen ratio (via carry trade and buyback)
The relationship between S&P and Russel2000 is more complex, even though Russel 2000 caught up in December rally (some say due to manipulations)
Russel 2000 is a more accurate indicator of american economy since its companies are less prone to carry trade and buyback mechanisms, but major players are interested in arbitrage between generic S&P500 and more specialized indices (Russel2000 [small cap value], Nasdaq100 [tech/bio growth], Dow30 [huge cap dividend]). The correlation between S&P500 and the world index (VTM) is currently strongly in favor of US, but recent actions of FOMC and ECB/Abe QE may change the balance of powers (VTM rally expected…)