Funding for this research was provided by:
Economic and Social Research Council (EC/R008841/1)
Received: 9 February 2020
Accepted: 22 May 2020
First Online: 8 June 2020
Ethics approval and consent to participate
: As our study is based on published or freely available statistical data and no data were sought from individuals, no ethical approval was required.
: The authors declare that they have no competing interests.