Think of it as an inverted pyramid, where you start with a wide overview but move towards the thesis statement or hypothesis, which should be the final element of the introduction.
In the introduction, you are attempting to inform the reader about the rationale behind the work, justifying why your work is an essential component of research in the field.
The introduction does not have a strict word limit, unlike the abstract, but it should be as concise as possible. It can be a tricky part of the paper to write, so many scientists and researchers prefer to write it last, ensuring that they miss no major points.
For a longer research paper, where you use an outline, it can be useful to structure your introduction around the outline. Here are a few outline examples.
The introduction gives an overall review of the paper, but does address a few slightly different issues from the abstract.
It works upon the principle of introducing the topic of the paper and setting it into a broad context, gradually narrowing down to a research problem, thesis and hypothesis. A good introduction explains how you mean to solve the research problem, and creates ‘leads’ to make the reader want to delve further into your work.
You should assume that your paper is aimed at someone with a good working knowledge of your particular field.
For example, a paper about evolutionary adaptations need not go into too much detail about Darwin - it is fairly common knowledge.
A behavioral science paper only needs to mention Pavlov and Skinner in passing, as their theories are standard for any first year undergraduate.
Like in any good Hollywood movie, the first task of the introduction is to set the scene, giving your paper a context and seeing how it fits in with previous research in the field.
Whilst not the...