In scientific publishing, authorship is the currency through which scientists are judged. Authorship is one of the most (if not the most) important factors in grant approvals, tenure decisions, and status in the field.
It is, first and foremost, important to be an author on a paper. However, not all placements are the same. So, of equal importance is where in the list of authors your name appears.
The two most important placements are the first author and the last author. Everyone in the middle of those two play a supporting role.
The last author is the head of the lab - the person who holds the grant and, therefore, funded the work. The first author did the most work on the paper - most likely the graduate student or post doc who thought of the project, designed the experiments and did them and maybe even wrote the paper. It's not uncommon for the first author to have done almost 100% of the work.
What if there are two people who split the work evenly? The first author position is so important for a scientist's career, no one who thinks that they deserve it is going to relinquish it easily.
There is a solution. Two (or more) people can split the first author position by becoming co-authors. In this case, the two names lead the list of authors and a small symbol is put above each with a notation stating that "these two authors contributed equally."
The tricky part of this is that someone's name still has to come first. Even with co-authorship, there still has to be a leading co-author. I have heard of people making the choice alphabetically or by seniority. But, other than that, it's a coin toss.
A group at UPenn wanted to look into many cases (2250 to be exact) of co-authorship and see if gender may play a role in whose name came first. The results are published in JAMA Network this week.
The group analyzed 862 cases of paper with co-authors with different genders. The papers were broken up into two groups, scientific and clinical, and analyzed separately. For scientific papers, the results are encouraging. The proportion of female authors listed first in the byline was equal to men (0.50.)
However, the gender equality did not hold up in clinical journals where the proportion of female authors listed first was 0.37. Further analysis showed that a female last author was associated with a female co-first author listed first in the byline.
So, there are probably lots of reasons why one co-author is put ahead of the other. Each case is different and driven by different factors. But, at least in the scientific world of publishing, gender does not seem to be a big factor. Unfortunately, the same cannot be said in the clinical publishing world and it's time for them to take a deep look into why women are coming second to men more times than not.
Source: Aakhus E, Gender and Byline Placement of Co-first Authors in Clinical and Basic Science Journals With High Impact Factors. JAMA. 2018;319(6):610–611. doi:10.1001/jama.2017.18672