If the author is an established expert in their field - and the paper is a new concept/theory then that is fine. Also - if it is an invited paper (usually for the same reason) - such as an editorial, review or continuous professional development (CPD) - then that is also fine. If it is primary research (and even secondary i.e. systematic review/meta-analysis), whether it is qualitative or quantitative, then single authorship should be avoided. As a reviewer, I am very wary of any primary research submission, if I know/can guess, that it is a single author. There is too much potential for data bias and manipulation without co-author member/cross-checking of the data. If the paper is submitted by a Higher Degree Research student - there should still be at least a minimum of one supervisor as an author as well.
If the author is an established expert in their field - and the paper is a new concept/theory then that is fine. Also - if it is an invited paper (usually for the same reason) - such as an editorial, review or continuous professional development (CPD) - then that is also fine. If it is primary research (and even secondary i.e. systematic review/meta-analysis), whether it is qualitative or quantitative, then single authorship should be avoided. As a reviewer, I am very wary of any primary research submission, if I know/can guess, that it is a single author. There is too much potential for data bias and manipulation without co-author member/cross-checking of the data. If the paper is submitted by a Higher Degree Research student - there should still be at least a minimum of one supervisor as an author as well.
Certainly in my main fields--political philosophy and political science--you want to see single authorship. To push the limits here you need to be free to explore and expose as you wish. It's for the reader to judge the outcomes.