- Correspondence to Professor John Henderson, School of Social and Community Medicine, ALSPAC, University of Bristol, Oakfield House, Bristol BS8 2BN, UK;
- Received 31 December 2014
- Revised 8 January 2015
- Accepted 14 January 2015
- Published in print 1 March 2015.
- Published online 16 February 2015.
- Published first 4 February 2015.
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