Did you know that we are automatically prone to judge good-looking people as more intelligent, honest and pleasant than less good-looking people? This is an example of what social psychologists call ‘the halo effect', and it takes any number of guises.
The halo effect refers to the basic human tendency to make specific inferences on the basis of an overall impression. Everybody knows that ‘first impressions count', but psychological research shows that the overall impression we receive of something — be it a person, business, institution or piece of work — exercises a hugely disproportionate influence upon our more specific evaluations of their merits, behaviour and performance.
However much university lecturers may pride themselves on their objectivity in evaluating student essays, psychological research shows that they are just as prone to such cognitive biases as the rest of us. Thus, although lecturers and examiners may believe they are evaluating a piece of writing solely in terms of its intrinsic merits, what they are really doing is your making inferences about the intellectual competence of the student who wrote it.
This is not to say that examiners won't take into account how well you have addressed the question set, or drawn upon the relevant literature in doing so. However, they will also be influenced by the impression they receive of the work as a whole. While the overall quality of the written prose may or may not enter into the explicit criteria they refer to when assigning you a grade, it's sure to influence their overall impression of your academic merit. And this overall impression, in turn, will be reflected in the grade you receive.
Thus, no matter how well you may have researched a topic, if you submit work containing spelling mistakes, typographical errors, poor grammar and cumbersome prose, this will inevitably create an unfavourable overall impression that will adversely affect your grade.
The take-home message is that you can be as intelligent as you like, but it has to shine through in your writing. If you write badly, with poor grammar and sentence construction, your examiner will form an unfavourable impression of your academic merit that will lose you valuable marks. It might well be the reason why that all-important upper-second or first-class grade keeps eluding you.