Received: 14 November 2019
Accepted: 2 January 2020
First Online: 11 January 2020
Compliance with ethical standards
: Rainer J. Klement, Nanina Brehm, and Reinhart A. Sweeney declare that they have no conflict of interest.
: This review was undertaken without requesting ethical approval, since ethical approval was granted for most of the included studies. All human data were anonymous, and the ethical recommendations of the Declaration of Helsinki were adhered to.