Received: 28 February 2018
Accepted: 18 March 2019
First Online: 11 April 2019
Ethics approval and consent to participate
: The study outline was approved by the <i>Ethics Committee at the University of Lund</i> in 1994 and 1998. Informed written consent was obtained from parents prior to participation.
: Not applicable.
: The authors declare that they have no competing interests.
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