The simple answer is that there's no common practice - which explains why you find large variation in how authorship order is chosen.

I've heard of practices where simple alphabetic order is used (remind me not to collaborate with colleagues who's surnames start with A ;-) and various practices involving the plastering names of heads of departments and such onto papers.

In my field on natural sciences, real importance is only given to the first and last author.

The importance of the first authorship is easy to understand as papers are cited and thus identified by the first author. However, for becoming the first author, our criteria are that the person has had a leading role in carrying our the research and has taken main responsibility for the writing of the paper, certainly written the first draft. This falls naturally on PhD students and postdocs and is a key step to their scientific career and recognition; mine was Björnsson et al 1983 :-). 

While I've seen a practice where the rest of the coauthors have been ordered in falling order of contribution to the study, so that the person contributing the least was the last author, this is definitely an exception. The rule is that the last authorship is reserved for the "leading senior scientist" often the head to the research group, i.e. the person who has come up with the research project concept, written the grant proposal which funds the project, has been involved in the experimental design and has the final say over the manuscript text.

The simple answer is that there's no common practice - which explains why you find large variation in how authorship order is chosen.

I've heard of practices where simple alphabetic order is used (remind me not to collaborate with colleagues who's surnames start with A ;-) and various practices involving the plastering names of heads of departments and such onto papers.

In my field on natural sciences, real importance is only given to the first and last author.

The importance of the first authorship is easy to understand as papers are cited and thus identified by the first author. However, for becoming the first author, our criteria are that the person has had a leading role in carrying our the research and has taken main responsibility for the writing of the paper, certainly written the first draft. This falls naturally on PhD students and postdocs and is a key step to their scientific career and recognition; mine was Björnsson et al 1983 :-). 

While I've seen a practice where the rest of the coauthors have been ordered in falling order of contribution to the study, so that the person contributing the least was the last author, this is definitely an exception. The rule is that the last authorship is reserved for the "leading senior scientist" often the head to the research group, i.e. the person who has come up with the research project concept, written the grant proposal which funds the project, has been involved in the experimental design and has the final say over the manuscript text.

Thank you very much Björn Thrandur Björnsson for your ...

Most journals have their own guidelines for order of authorship.

You may also check the thread linked below where this topic is discussed with over 100 comments.

https://www.researchgate.net/post/Should_professors_jointly_publish_papers_with_students?_tpcectx=qa_overview_asked&_trid=54d6631ad2fd6491488b456f_

Dear Dr Tulsi Bhandari,

I agree very much with Dr Björn Thrandur Björnsson.

In our limited experience also, one who contribute  more should be the first author, even though it is our students. Usually one who just review and proof the version, is at the last.

Hi John F. Wilhite,

In which field of science do you see journals dictating the author order? I'm just curious because I've never seen any explicit journal instruction on this in my field which in comparative/fish physiology and endocrinology. Cheers.

Bjorn,

Nicholas Rowe lists a number of sources on page 16 of the above-cited thread.  These would provide some general guidelines since there are no specific guidelines in the field of fish physiology.  In the latter case it seems it would be up to the authors to decide.

Regards, John

Thank you all for your sharing...

In my experience the issue of the order of authors has very different  meanings in different subject areas.  In social science and humanities, though, 'normal practice' does exist in my experience in terms of the expectations I have come across: alphabetical order is considered usual.  The exception is where the author team agrees that someone has made an exceptional contribution in which case they come first.  Those are the expectations in social science among everyone I have come across in forty years publishing in these areas.

Of course, it's all about experience and norms so everyone is entitled to a view!