There are many ways to structure an introduction. However, in the context of psychology I am familiar with the two broad approaches you mention to writing an introduction.
In psychology, students are often taught to write lab reports using an hour glass structure where they start broad and then narrow in gradually into stating aims and hypotheses at the end of the introduction. This structure can really annoying to read, because the purpose of the paper is not immediately clear.
Personally, I prefer the opening-body-currentstudy structure for an introduction.
Specifically, the opening section of the introduction contains around three paragraphs that cover the importance of the work, a little context, a little bit of the gap, and importantly the aims of the paper. The introduction then reviews relevant literature, and culminates in a statement of a brief description of the current study (see here for a little more discussion). Thus, in the opening few paragraphs, you might have a paragraph that begins: "The purpose of this paper is..."
The general principle is: Make it easy for the reviewer/reader to see what is the novel contribution of the paper.