There has been numerous debates since the start of this year regarding the rise of US stocks eg S&P 500.Many "experts" think that S&P is very expensive now relative to historical data. They used Robert Schiller CAPE index which now stand at 25. This was exceeded only three times since 1881 in 1929, 1999 and 2008. They preceded the Great Depression, dotcom burst and GFC respectively.Historical the CAPE for S&P run around 15.21 in the 20th century.
However, there is another camp that look at the data differently. To them, the S&P index PE is at 16.2 based on 2014 earnings. S&P earning per share is at $120. You typically need to get to PE at around 20 for sign of bubbles.So, to them, there are still a lot of legs of the current bull run.
What is my take? frankly, I do not know because I am not an expert and experts made all the wrong calls most of the time anyway. However, I do know of two things. The S&P now rely on a lot of overseas earnings compare to the previous decades. The emerging countries in particular China has grown by leaps and bounds and are now very important to S&P earnings. That is probably the reason why since 2009, Main Street is not doing well compare to Wall Street in the US. Main Street focus on the local US economy whereas Wall Street focus on corporate profits. Next, is that the low interest rate environment do push down the cost of debts and drive up earnings.