What explains the increase in trade union density and female share of union members in the United Kingdom in 2017–2020?

Trade union density increased for three consecutive years in the United Kingdom between 2017 and 2020. This contrasts with a general decline in union membership since 1979. Since union density continued to fall amongst male employees in 2017–2020, the overall increase was entirely attributable to females. This paper explores the factors which explain why there was an increase in overall union density after a period of decline (for males and females) and why union density evolved so differently for males and females. Using decomposition methodologies, we find that the increase in union density in 2017–2020 was not due to a higher propensity to unionise within particular groups or across all employees. Instead, the principal driver of the overall rise in 2017–2020 was an increase in the proportion of employment in certain public sector organisations. The largest contributor to the difference across males and females was increases in the share of employment in more unionised occupations amongst female employees and decreases amongst male employees.