In academic essays, a strong paragraph spans several sentences, but tackles only one central idea. While there's no such thing as the correct number of sentences in a paragraph, a paragraph can be too long or too short. It all depends on what purpose the paragraph is supposed to serve in the writing project.
While there's nothing inherently wrong with very long paragraphs, they're often a sign that more than one idea is being discussed, so they should be broken up into two or more paragraphs. Also, an overly long paragraph puts a strain on readers.
Breaks between paragraphs are used as places to rest while reading— places to process what is read, and to prepare to continue on through the text. Therefore, it should make sense that not giving your readers enough of these breaks will hurt your essay's readability, even if you don't change a word in it.
On the other hand, having paragraphs that are too short is also usually a problem. Again, there's nothing inherently bad about them, but short paragraphs are often a sign that an idea has not been completely developed, or that one fully realized paragraph has been arbitrarily divided into two.
For readers, too-short paragraphs can be overlooked amongst longer ones; or, when they come in numbers, they can create a jittery, jarring reading experience, which is arguably just as bad as having no paragraph breaks at all. Besides, if the purposes of a paragraph are to have a central claim and to support it, then a paragraph should be long enough to do both.
As with the sentences in a paragraph, there is no correct number of paragraphs for an essay; the number should reflect the needs of the text itself and the author's goals, which are always going to vary from text to text and from person to person.
In order to discuss more clearly the form and function of a paragraph, it's important to discuss its key parts. Nothing is more central to an academically-sound paragraph than its topic sentence. This is the sentence that most clearly expresses the thesis of a paragraph.
In many ways, the paragraph is like a miniature essay, and like an essay, a paragraph's thesis often comes at or near the beginning. It is often, but by no means always, the topic sentence that starts a new paragraph, with all the rest following it as support.
Having focused paragraphs with solid topic sentences helps to anchor readers within the text and make it easier for them to absorb the writer's ideas rather than having to pay attention to how those ideas are being displayed.
For the writer, having focused paragraphs with solid sentences is also useful, as during the revision and editing process it's easier for the writer to look at each paragraph and see whether or not the single idea it articulates supports the main thesis of the essay. This question is much easier to answer - whether yes or no - when the paragraph itself is clearly focused.
Now, if a topic sentence is the central thesis of the paragraph, what does that make all the other sentences? In composition, they are referred to as supporting sentences. These are the sentences in a paragraph that support its thesis, or main idea. In this way a paragraph is, again, like a miniature essay.
Supporting sentences exist to explain or demonstrate the truth of the topic sentence, and they can do so through the expression of ideas, facts, data, logic, or other means of intellectual support.
When writing, it's important to pay attention to what each supporting sentence is contributing to the paragraph as a whole. It's equally important to cut, change, or relocate irrelevant sentences - any that don't do enough to support the thesis - even if they are interesting or relevant elsewhere, or for other purposes.
Now, let's take some time to look at a couple of paragraphs in order to see how topic and supporting sentences work (or don't work) together.
Read the paragraph below. As you do, look for the topic sentence and any supporting sentences.
Commensalism is the name for a biological interaction in which one organism benefits and the other derives neither benefit nor harm. In a stable environment - like that which occurs in isolated environments - organisms evolve to lose anything that isn't necessary to their daily needs. But when that environment changes, when the organisms upon which others depend can no longer survive, entire ecosystems can crash. Though much less commonly known than the related terms of parasitism and symbiosis, commensalism is common when multiple species live together in non-predator-prey relationships.
So, what did you find? The first sentence clearly seems to be the topic sentence. But what about the second sentence? It doesn't really seem to be supporting the first— or at least, not supporting it very well. Also, the third sentence seems to be supporting the second sentence more than the first, so perhaps that's the topic sentence? The fourth and last sentence is definitely a supporting sentence here, but it could be its own topic sentence in some other paragraph.
In any case, if, as you were reading this, you got a little confused, don't worry. This paragraph was chosen as an example because it demonstrates what happens when a paragraph doesn't have a clearly stated central topic. This one seems to have two, or maybe even three. The writer would certainly benefit from a little revision time, and should probably work a little harder to make sure she knows exactly what this paragraph wants to say and why.
Let's look at another paragraph— one that won't cause so much confusion. It contains some fairly complex ideas, so take your time reading it. It is an excerpt from an essay titled "Reconstruction" by Frederick Douglass, a writer, social reformer, statesman, and escaped slave.
There is cause to be thankful even for rebellion. It is an impressive teacher, though a stern and terrible one. In both characters it has come to us, and it was perhaps needed in both. It is an instructor never a day before its time, for it comes only when all other means of progress and enlightenment have failed. Whether the oppressed and despairing bondman, no longer able to repress his deep yearnings for manhood, or the tyrant, in his pride and impatience, takes the initiative, and strikes the blow for a firmer hold and a longer lease of oppression, the result is the same: society is instructed, or may be.
This paragraph, while full of complicated ideas and a nuanced approach to them, should still have been easier to grasp. Did you find the topic sentence? It's the first sentence of the paragraph. You can hopefully see how every other sentence in the paragraph supports it, either by explaining why we should be grateful, or going into detail as to what about rebellion gives us cause to be grateful.
Did you notice the slightly circular form here, how the first ("It is an impressive...") and last ("society is instructed...") supporting sentences seem to come back around to the same point? By the end, we readers are in a much better position to appreciate why or how rebellion is a good teacher. As mentioned above, the paragraph is like a miniature essay, almost self-contained in this case.
Finally, let's look at one more paragraph - a very different, and in many ways, much simpler, paragraph - that should also help demonstrate how topic and supporting sentences can work together.
During the months that followed the Japanese bombing of Pearl Harbor, everything changed for Hawaii. Martial law was declared and the radio carried stories of internment camps forming in California, though of course no general was dumb enough to try that in the islands. Japanese women stopped wearing their kimonos to the market anyway, and even my great grandmother, descendant of Irish farmers that she was, began keeping her own kimono, a gift from a former maid, in the back closet.
Here, again, the first sentence is the topic sentence, expressing the paragraph's thesis. Even though the paragraph is much more narrative, the argument it makes in its topic sentence - that everything changed - is supported by the information contained in all the rest of the sentences. Whether historical, cultural, or personal, we're still being given supporting evidence about the change.
Now that you've seen three different ways sentences can interact in a paragraph, you should be in a better position to read other paragraphs and to write your own.