Implications of using simple past instead of present perfect

Question

I am reading a text (a PhD thesis) which almost exclusively uses simple past tense when describing something that has happened in the past. In some cases, I feel that the author should have used past perfect, and using simple past leaves a completely wrong impression, for example (I have changed the citations a little because I would not like the author or the topic to be identifiable -- i.e. the original was not about bubble gum):

Research on bubble gum consumption also showed a link between chewing frequency and digestion problems (Jenkki & Orbitsdottir, 2018).

Here, the author seems to suggest that she has studied this link herself -- but she didn't, because this is her first study, and the citation at the end of the sentence makes it clear that she is referring to other people's research. My intuition (but I am not a native speaker) tells me that in this case, one should use present perfect -- is that correct? That is, the sentence should be:

Research on bubble gum consumption has also shown a link between chewing frequency and digestion problems (Jenkki & Orbitsdottir, 2018).

Another example: at the end of a chapter, the author mentions differences in chewing gum consumption between Eastern and Western European countries, and then says:

Several theories were proposed to explain the possible mechanism of the differences (Otros & Aliae, 1999).

My unreflected impression is that she is referring to the theories she has proposed earlier in that chapter. But there was nothing like this in the chapter, and the citation, again, makes it clear that she is referring to others' research. So I would re-write this sentence as:

Several theories have been proposed to explain the possible mechanism of the differences (Otros & Aliae, 1999).

Is my interpretation correct?

To be clear, I am not asking about grammatical correctness -- I suppose that either tense can be used in these sentences. My question is about the nuances of meaning: is it correct that he original sentences imply that the author is talking about her own recent work, something that was just discussed on the preceding pages?

Answer 1

I don't get the sense that the author conducted the research herself. But you've not given much to go by.

If it's unclear where the research comes from throughout the paper, the bigger issue is the lack of a definite article for research (research, as opposed to the research).

Generally, in these types of papers, writers will call what work has already been done on the topic "the research." With little context, and nothing that explicitly indicates to whom this research belongs, the first sentence you provided, as you suggest, is ambiguous. However, so is your alternative, which gives the impression the research is ongoing. This could also be true of research conducted by the author.