by Nicole Kagan and Nicolas Yan
What are topic sentences?
Topic sentences are sentences that lead each paragraph of an essay. Think of them as flags: at the beginning of each paragraph, you are “flagging” to the reader what the direction of the paragraph will be (more concretely, what the claim you are making in that paragraph is).
Why are topic sentences important?
Topic sentences are important both for the reader and for the writer. Because topic sentences clearly present arguments in a way that flows, they allow a reader to understand not only the direction of each paragraph, but also the snowball structure of the paragraphs taken together. That is, before you can prove Y (your thesis), you must first prove X1, X2, and X3 (every claim made in the body paragraphs). For the writer, meanwhile, topic and supporting sentences are essential for conceptually and specifically outlining the structure of the essay and increasing its clarity.
Paying attention to topic sentences can often deliver the highest return on investment when it comes to editing your paper, especially if you don’t have much time. As a general rule of thumb, somebody reading your essay should be able to gain a good understanding of what your argument is about by simply reading the introduction, each topic sentence, and the conclusion. By going through this “skim-reading” exercise and revisiting your topic sentences, you should be able to identify any logical or structural weaknesses in your argument, particularly with regards to the order of your paragraphs.
So what makes for an effective topic sentence?
An effective topic sentence should accomplish four main functions:
- It should highlight the claim (i.e., the main idea) of the paragraph that follows;
- It should organize and advance your argument by relating back to your thesis statement and developing your argument’s logical progression;
- It should create signposts that enable the reader to easily follow your argument, often by utilizing bridge words (see this blog post for more on bridge words); and
- It should be specific—if it would be possible to pick up your topic sentence and drop it into another essay with a different thesis, then it is not specific enough.
To illustrate these functions more clearly, let’s take a look at the topic sentences of an example essay.
Imagine that you have been asked to discuss whether the most effective way of reducing gas consumption is providing government subsidies for electric vehicles. You might decide to argue this thesis:
Although subsidizing electric vehicles would help to reduce gas consumption, an effective government strategy must also boost the demand of electric vehicles relative to gas-powered cars by instituting a gas tax.
Your argument might proceed according to the following steps:
- First, you might begin with a general explanation of how subsidies work.
- Next, you might explain how subsidizing electric vehicle production would reduce gas consumption.
- Then, you might concede that pursuing a strategy of government subsidies in isolation may have some drawbacks.
- Next, you might argue that governments should simultaneously pair subsidies for electric vehicle production with a tax on gas.
- Finally, you might want to address a potential counterargument.
Your topic sentences should therefore mirror the structure of your argument:
- TS1: To understand this argument, it’s important to understand that government subsidies work by encouraging companies to increase the production of a particular good by lowering production costs.
- TS2: Government subsidies for electric vehicles would reduce gas consumption by increasing the supply, and decreasing the price, of electric vehicles.
- TS3: However, insofar as subsidizing electric vehicles would only target the supply of electric vehicles, pursuing such a strategy in isolation would have certain drawbacks.
- TS4: Therefore, in order to target both the supply of and the demand for electric vehicles, governments should simultaneously pair electric vehicle subsidies with a gas tax.
- TS5: Though some might argue that subsidizing public transportation would be a more effective way of reducing gas consumption, such a policy would overlook those who live in more rural areas that lack connectivity to public transportation networks.
Each of these example topic sentences clearly highlights the claim, or main idea, of the paragraph that will follow. Additionally, these sentences clearly organize and advance the argument—each topic sentence logically builds upon the previous one. The argument would not make sense if the topic sentences were rearranged (for example, TS4 could not precede TS3, since the claim in TS4 logically depends on the claim in TS3). Furthermore, using signpost words (like “however,” and “therefore”) helps to signal transitions between ideas to the reader. And perhaps most important, these topic sentences make specific claims that would not easily fit into a different argument. Finally, they pass the “skim-reading” test; reading each of the topic sentences would give the reader a pretty good idea of the overall argument, even without the body paragraphs.