From the WSJ.

SPX slightly above 17x forward earnings (2043), so interesting to see if this level holds. INDU at 16.5 so if that goes for 17x means target 18150 area. Lastly NDX just above 18x (4308) so that is another level to watch as possible support. 

Sum: Above SPX 2043 and NDX 4308 adds to bullish case, means managers willing to pay up 17x for SPX and 18x NDX. Falling back under these levels would add to bear case.

At this point I expect resistance at SPX 18x currently 2163 which very conveniently lines up with YR1 also exactly at 2163!