Since the Digital Object Identifier (DOI) system was introduced in 2000, guidelines on how to best display DOIs have evolved. Effective March 2017, Crossref, an influential DOI registration agency, now recommends the following format:

https://doi.org/10.1037/rmh0000008

Note the use of “https” instead of “http”, and “doi.org” instead of “dx.doi.org”.

While the 6th edition of the Publication Manual of the American Psychological Association, published in 2010, recommended the original doi:10.1037/rmh0000008 format, APA has updated their guidelines to follow the Crossref guidelines for displaying DOIs with this latest format.

The CSL styles for the 6th edition of APA has therefore been updated to use the https://doi.org/10.1037/rmh0000008 format as well.

As Crossref explains in their guidelines, the original concise doi:10.1037/rmh0000008 format was recommended with the hope that web browsers would one day automatically recognize and hyperlink these DOIs.

Since this didn't happen, Crossref changed its recommendation in 2011 to show DOIs as regular URLs instead, in the format http://dx.doi.org/10.1037/rmh0000008. APA started to recommend this newer (but nowadays also outdated) format in its 2012 companion guide, APA Style Guide to Electronic References, and discussed this change in a 2014 blog post.

More recently, there has been a strong movement to move the web over from HTTP to the more secure HTTPS protocol. Technical changes also made it possible to link DOIs via the shorter doi.org instead of dx.doi.org. Together, this let Crossref change its recommended format to https://doi.org/10.1037/rmh0000008.