75 per cent of U.S industries have become condensed with a small number of leading firms in each sector such as Microsoft, Apple, Amazon and Facebook. These firms dominate their respective industries in terms of sales, profits and equity returns. So why there is a growing concern for investors backing these so called ‘U.S superstar firms’. Joining CNBC Africa for more is Gavin Ralston the Head of Official Institutions and Thought Leadership at global asset manager Schroders.