It is known that generally the definite article is not used with names of people. However, when the names are preceded by names of occupations then, as some sources say there must be used "the". For example: "The psychologist Mike Smith is going to deliver a speech." But some English speakers say that the article should be omitted. For instance: "Author Conor MacGregor is coming on" not "the author Conor MacGregor is coming on." So, could anyone tell me what variant is grammatical to use with "the" or without?
Your sources are correct; it should be: "The psychologist Mike Smith is going to deliver a speech."
Although using the definite article in this case sometimes sounds odd to American ears, when it is not a real title you should include it. This has to do with the concept of "false title". You can use the "Good morning" test, to determine the correct usage:
- Good morning, Dr. Jones. ✔ → Dr. Jones is going to deliver a speech. ✔
- Good morning, psychologist Smith. ✘ → Psychologist Smith is going to... ✘
The first one is okay, because "Doctor" is a real title; thus "Doctor Smith" does not require the definite article (indeed, must not have it). The second one, although acceptable to some, especially in AE, is not correct, because "pscyhologist" is a false title.
Linguists call this an anarthrous occupational nominal premodifier. The best analysis of this is by Greg Pullum at Language Log (2004-11-07).
I think it depends on the context. If the person is well known, I would write
The psychologist Mike Smith is going to deliver a speech.
This indicates that Mike Smith is the famous psychologist, not another Mike Smith. But if introducing someone who is not well known, I would skip the definite article.
Author Conor MacGregor is coming on.
In this case it advises us what Conor does, but he is not yet a household name.