Different disciplines follow different conventions regarding verb tense.
Historians and literary critics, for example, conventionally refer to historical and literary narrative in the present tense. Here is an example of writing in the "historical present" from Professor Bill Cook's writing guide:
When Pericles speaks to the Athenian Assembly, he convinces the majority that they should go to war against Sparta. He predicts, however, that they will lose the war unless they act prudently.
In psychology, writers typically review previous literature and report results in the past tense. They state hypotheses and well-accepted findings in the present tense. The important thing is to keep verb tenses consistent. In general, you should not switch between present tense and past tense in the middle of a paragraph. When a particular distinction must be made about time sequence, common sense will dictate when to switch.