When it comes to annual evaluations and consideration for merit pay increases, not being first author on a coauthored academic paper can be an issue between evaluator and the person being evaluated. Being last author on too many coauthored papers can hurt tenure chances or promotion from associate professor to full professor.
There seems to be no set standard, however, as to what constitutes a universal justification for being first author on an academic paper--or for being last author.
Therefore, what justifies being first author on a coauthored academic paper?