How common are single-author research articles in cognitive psychology? Are single-author research articles looked down upon?
To target the question in the question body here:

This question is quite opinion-based, because everyone can decide to frown upon something. However, I think that, in general, it's much more about:


Who the author is, i.e., what is their track record in the field;
What their affiliation is;
What the quality of the article is;
What the quality (e.g. impact factor) of the journal is.


These considerations are, at least to me, much more important than how many authors follow or precede someone in the author list. I would never judge an article on the number of authors per se. Lastly, (invited) review papers are often authored by just one author, even in the STEM sciences.