Apple, Amazon, Alphabet and Microsoft now account for 15% of the S&P 500, or $3.8 trillion out of the S&P 500 market capitalisation of $25.91 trillion. The S&P 500 is up 9% this year compared to an average of the big four tech stocks up 47%, or between 20 to 73%. This is a chart doing the rounds on Reuters – it is an index of the big four tech stocks (orange) compared to the 20 month moving average of the index over the last 20 years. The gap between the 20-month moving average and the index is now at 44%, the highest in nine years. Last time the gap was this wide the index sold off more than 20% over the next seven months.