August turned out to be some month for stock markets. The S&P 500 was up 7 per cent as the month neared its end last week, hitting fresh all-time highs. The sheer consistency of the rally is even more impressive than the gains, says Bespoke Investment; the index advanced on over 80 per cent of days in August.

Making money is not meant to be this easy. Since 1993, there has never been a month with a higher percentage of up days, according to Bespoke. Indeed, there have only been six months where the percentage of up days exceeded 75 per cent.

Nevertheless, this picture of historic market strength is misleading. A separate Bespoke report found market breadth was negative – that is, more stocks declined than advanced – on eight of the first 16 trading days in August. The same pattern of strong market gains but weak breadth was evident in July and May of this year but this is very atypical action – there was not a single such instance in the previous decade.

What's going on? The five biggest technology stocks – Apple, Microsoft, Amazon, Google and Facebook – now account for a quarter of the S&P 500's total market capitalisation, with their enormous gains masking broader market weakness.