Why have the big seven tech companies been hit by AI boom doubts?

Their shares have fallen 11.8% from last month's peak but more AI breakthroughs may reassure investors

It has been tough week for the magnificent seven, the group of technology stocks that has played a dominant role in the US stock market, buoyed by investor excitement about breakthroughs in artificial intelligence.

Last year Microsoft, Amazon, Apple, the chipmaker Nvidia, Google's parent, Alphabet, Facebook's owner, Meta, and Elon Musk's Tesla accounted for half the gains in the S&P 500 share index. But doubts about the return on AI investment, along with a mixed set of quarterly results, investors shifting their focus to other sectors and weak US economic data have hit the group over the past month.