I know this is more an FX site, I chose the S&P trade, not to show off or be some sort of smart wise guy, I wanted to attempt to highlight the notion that the market is one place.
In the olden days there were spooze traders or currency guys or maybe the bond traders and so on. Most these traders specialized and they were good in their own arena, they had only a passing interest outside that field.
Those days are gone, caused by a number of things, some have blamed 'globalisation', others inter governmental trade treaties including the creation/expansion of the EU. My humble opinion is the introduction of electronic trading and this device that I am typing on, this combination caused the creation of the Algo .
The beauty of the algo is that it is focussed on the right side of the chart, it knows only one market, it is programmed to look for risk on or off, it has no sense in that it cannot feel sentiment, so it has to look to other markets for guidance.
Just check this out, right side of the chart, look closely at how the stock market, the USDX and Gold react this week ahead. Get a sense of who is leading and when the algos click in in these 3 markets
For the guys asking about the behaviour of the S&P vs FX, stock markets are very much more mood orientated, especially into the close, so if you see a peak late in NY and then price begins to fall, usually about an hour before close, then if it is late in the week most guys just sell, they don't want to be wrong side over the weekend just in case there was some sort of news that they don't know about but that others do.
Right side of the chart again, tomorrow, maybe after one or two hours trading the S&P will likely rise, then again, always remember that right side is seldom timing exact, best you can do, as Shop says, think in probabilities - are US companies in difficulties or not.