Chart 1 plots 4  the turnover share of the largest 100 UK businesses since 1998 (i.e. concentration ratio). This ticks up in the lead-up to the financial crisis, although this pick-up is more modest than in the US, from 20% to around 5  28%. Concentration has flattened-off in the period since the crisis, however, in line with other European countries. Turning to measures of concentration within the financial services industry, the international pattern is somewhat more uniform. Chart 2 plots the largest five banks’ share of total banking assets in the US, euro area and the UK. Levels of banking concentration started fairly high, averaging around 30%. They drifted further upwards in the run-up to the crisis, although this drift was again fairly modest. Since the crisis, however, measures of banking concentration have flat-lined and, in the UK, have fallen slightly. Concentration indices have their limitations, though, and need not always be associated with market power. Some firms may be able to exercise market power in setting prices even without having a large share of a market if, for example, there is brand loyalty. And in a world of differentiated products, concentration measures such as HHIs or concentration ratios no longer correlate closely with market power (Bresnahan (1989)). With non-homogenous goods and non-Cournot competition, a better measure of market power is often provided by firms’ mark-ups – the ratio of their price to their marginal cost (De Loecker and Eeckhout (2018)).
14  Within that, this rise in mark-ups among foreign relative to domestic sales-focussed firms is largest in the manufacturing and ICT sectors. Taken together, this evidence is consistent with a story of rising mark-ups being concentrated among internationally-operating firms, who perhaps benefit disproportionately from global network economies of scale and scope. These firms tend to occupy the fat and fattening upper tail of the mark-up distribution. These are firms that might legitimately be termed “superstars” (Autor et al (2017)). Given this diagnosis, what impact might the rise in mark-ups have had on the macro-economy? One aspect is what impact increased market power may have had on firms’ incentives to invest and innovate and hence on firms’ productivity. With investment and productivity each having under-performed over recent years, the relationship with market power has been subject to increased academic scrutiny recently (for example, Eggertsson, Robbins and Wold (2018)). The relationship between mark-ups and productivity is vital in understanding their macro-economic effects (Van Reenen (2018)). On the one hand, if highly productive ‘superstar’ firms, benefitting from network economies of scale and scope, have become more dominant, then higher mark-ups could be the side-effect of a positive supply shock in the economy. On the other hand, if mark-ups are the counterpart to increased market power and reduced competitive pressures, that would suggest a negative supply shock. These effects are not mutually exclusive. For example, Aghion et al (2005) develop a model which generates a concave relationship between competition and investment.