US stocks are nearing February's all-time highs but the narrowness of this rally is a growing concern. Goldman Sachs last week noted the five largest stocks – Apple, Amazon, Microsoft, Google and Facebook – have gained 35 per cent this year, whereas the remaining 495 stocks in the index have fallen 5 per cent.

Those five tech stocks account for 22 per cent of the index. On average, the five biggest stocks have historically accounted for only 14 per cent of the index. Today’s market is even more concentrated than the late 1990s technology bubble, when large-cap tech stock strength kept indices aloft even as the rally grew ever more narrow. Although the S&P 500 was last week within 5 per cent of 52-week highs, more than 60 per cent of the index is over 10 per cent below their own highs – something that hasn’t happened since September 2000, according to SentimenTrader data.

On Monday last week, the S&P 500 enjoyed decent gains but there were more decliners than advancers on the New York Stock Exchange (NYSE). That has only happened on five occasions, says SentimentTrader, all of which were in 2000 or 2008.

Similarly, it notes that while the Nasdaq gained over 2 per cent, only around half its stocks rose that day – a combination unseen since February 2000, just prior to the bursting of the tech bubble. Investors will be hoping market breadth improves. A narrow market rally is a fragile market rally.