In mathematics and several other areas of research, there is a tradition of alphabetizing author lists. Furthermore, most good multi-author papers are truly joint work. As the American Mathematical Society stated in "The Culture of Research and Scholarship in Mathematics: Joint Research and Its Publication":

In most areas of mathematics, joint research is a sharing of ideas and skills that cannot be attributed to the individuals separately. ... Determining which person contributed which ideas is often meaningless because the ideas grow from complex discussions among all partners. ... For this reason, mathematicians traditionally list authors on joint papers in alphabetical order.

Consequently,
**the order of authors says nothing about who was "lead" author,**
and for many papers
**the concept of a "lead" author
does not make sense**.

In recognition of the fact
that author lists are often alphabetized,
I **always cite the full author lists for all papers**,
and I require my coauthors to follow the same rule.
This requirement applies to all citations
(bibliography entries, textual credits, parenthetical notes, etc.),
whether in papers or in slides.

By default, my papers follow the mathematical tradition of listing authors in alphabetical order. I have begun adding an explicit footnote to the first page of every paper saying "Author list in alphabetical order; see https://www.ams.org/profession/leaders/culture/CultureStatement04.pdf."

On occasion it is clear that an individual author is responsible for an exceptionally important contribution to a joint paper. Research teams can decide to make this fact public, either in the paper itself or in other venues; and senior authors are expected to report junior authors' contributions in detailed recommendation letters. However, reordering an author list is a low-bandwidth, noisy, superficial, error-prone method of attempting to communicate this information.

I will make an exception to the mathematical tradition for coauthors who, at the beginning of the collaboration, show me that they have published a policy of listing authors in decreasing order of contribution.

No matter what author order is used,
listing someone as an author is stating
that the author made an important contribution to the work.
**Honorary authorship is prohibited under all circumstances.**
This does not necessarily mean
that all listed authors
participated in *writing* the paper;
writing the paper is only one part of the work.