As a rule of thumb for social science writing, on-going issues and current realities should be reported in present tense. What has occurred, what has been reported, and what you and others have accomplished should be reported in past tense. That is, if health care costs no longer are a problem, or if oil imports are no longer increasing, past tense works. Otherwise, as on-going realities or generalizations about them, they should be described in present tense. In that case, presumably other researchers discovered other reasons, or the results of Jenkins's study couldn't be generalized.
Some writers refer to past research in present tense.
Show, don't tell. Boris saw that someone had hit his car and felt really upset. Without a doubt, Head Start has been one of the lasting bright spots in President Johnson's War on Poverty. Before the advent of the computer, acquisition activities, that is, pre-order searching, typing of purchase orders, etc., were often performed by a crew of persons who seldom ever crossed the technical services function lines and performed, for example, processing activities, that is, typing order forms, filing sales receipts, etc.