Article History
Received: 27 July 2016
Revised: 28 April 2017
Accepted: 16 May 2017
First Online: 2 June 2017
Competing interests
: Professor Fitzsimons is the Principal Investigator of the Millennium Cohort Study, the data set used in this paper. However, this role is independent of the analysis presented in this paper. The other authors declare no conflict of interest.