Funding for this research was provided by:
National Institute on Drug Abuse (R01 DA033958)
National Institutes of Health (T32GM108557)
Article History
Received: 5 December 2016
Accepted: 30 June 2017
First Online: 24 July 2017
Ethics approval and consent to participate
: The Minnesota Center for Twins and Family Study was reviewed and approved by the University of Minnesota Humans Subjects Committee (Institutional Review Board).
: Not applicable.
: The authors declare that they have no competing interests.
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