AAPL now accounts for a full 4% of the value of the Standard & Poor’s 500, widely considered to be one of the best representations of both the US stock market and the broader American economy …

The WSJ notes that this is the highest percentage achieved by any company for a decade.

The paper’s James Mackintosh suggests that this could be cause for concern.

Normally, this kind of concentration of wealth in a small number of companies in a single sector is considered troubling, leaving the market as a whole vulnerable to any shift in their fortunes.

The piece notes that an investor who had put equal sums into each of the five companies a year ago would have made a 38% return even without factoring in dividend income.