Trade Union Membership Concentration, 1892-1987: Development and Causation

Existing studies of British trade union membership concentration have used the variance of logs and concentration ratio measures to show a tendency towards increasing concentration. This paper proposes the Herfindahl index as a more appropriate measure. The annual variation in concentration is analysed by means of the Herfindahl index and shows some fluctuation, particularly prior to 1947. Divergent trends are also shown for manual and white-collar membership concentration. An explanation of the aggregate trend and of these different occupational trends is advanced in terms of bargaining structure and the development of general unionism.