5.2: Topic Sentences

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Now that you have identified common purposes for writing, learned how to select appropriate content for a particular audience, and written a working thesis (at least) you can think about the structure of a paragraph in greater detail. Composing an effective paragraph requires a method similar to building a house. You may have the finest content, or materials, but if you do not arrange them in the correct order, then the final product will not hold together very well.

A strong paragraph contains three distinct components:

1. Topic sentence. The topic sentence is the main idea of the paragraph.
2. Body. The body is composed of the supporting sentences that develop the main point.
3. Conclusion. The conclusion is the final sentence that summarizes the main point.

The foundation of a good paragraph is the topic sentence, which expresses the main idea of the paragraph. The topic sentence relates to the thesis, or main point, of the essay and guides the reader by sign-posting what the paragraph is about. All the sentences in the rest of the paragraph should relate to the topic sentence.

Topic Sentences

In academic writing, readers expect each paragraph to have a sentence or two that captures its main point. They’re often called “topic sentences” or "main ideas." Don't be fooled by the phrase “topic sentence” though. It makes it seem like sentence's job is simply to announce the topic of the paragraph. Rather think about the topic sentence for the paragraph in the same way you think about the thesis statement for the essay as a whole (see "Thesis Statements"). Sometimes topic sentence will also include a transition from the previous paragraph, but not always. A topic sentence should never simply be a transition.

Consider these two examples: ²

Version A: Now we turn to the epidemiological evidence.

Version B: The epidemiological evidence provides compelling support for the hypothesis emerging from etiological studies.

Both versions convey a topic; it’s pretty easy to predict that the paragraph will be about epidemiological evidence, but only the second version establishes an argumentative point and puts it in context. The paragraph doesn’t just describe the epidemiological evidence; it shows how epidemiology is telling the same story as etiology. Similarly, while Version A doesn’t relate to anything in particular, Version B immediately suggests that the prior paragraph addresses the biological pathway (i.e. etiology) of a disease and that the new paragraph will bolster the emerging hypothesis with a different kind of evidence. As a reader, it’s easy to keep track of how the paragraph about cells and chemicals and such relates to the paragraph about populations in different places.

By clearly establishing an essential point within its analytic context, a well written topic sentence gives both you and your reader a firm grasp of how each point relates. For example, compare these two sets of topic sentences, each introducing a sequential paragraph³:

Set A:

At the beginning of the AIDS epidemic, the cause of the disease was unclear. …

The cause of AIDS is HIV.

There are skeptics who question whether HIV is the cause.

Set B:

At the beginning of the AIDS epidemic, the cause of the disease was unclear, leading to a broad range of scientific speculation.

By 1986 HIV had been isolated and found to correlate almost exactly with the incidence of AIDS.

HIV skeptics, on the other hand, sought to discredit claims based on epidemiology by emphasizing that the pathogenesis of HIV was still unknown.

Set A isn’t wrong per se; it just illustrates a lost opportunity to show the important connections among points. Both versions portray a process unfolding over time: initial uncertainty followed by a breakthrough discovery and then controversy. Even with the same substantive points, a person reading Version A would have to work harder to see how the material in the paragraphs connects. Readers experience Version B as clearer and more engaging.

Thinking of topic sentences as sequential points in an argument reminds one that a key sentence doesn’t have to always be a single declarative one. Sometimes you need two sentences together to achieve the work of a topic sentence, and sometimes a question or quotation does a better job than a declarative sentence in clarifying a logical sequence, which brings us to Set C.

Set C:

At the beginning of the AIDS epidemic the cause was unclear. Virologists, bacteriologists, immunologists, and epidemiologists all pursued different leads, reflecting their particular areas of expertise.

If drug use, lifestyle, and “immune overload” didn’t cause AIDS, what did?

“I’ve asked questions they apparently can’t answer,” claimed retrovirologist Peter Duesberg⁴ who became an oft-quoted skeptical voice in media accounts of AIDS research in the mid-1980s.

Set C is based on the same three sequential points as Sets A and B: (1) the cause of AIDS was initially unclear (2) HIV was accepted as the cause (3) lone dissenters questioned the claims. However, sets B and C have much more meaning and momentum, and set C, depending on the nature of the argument, features more precise and lively stylistic choices. Opening the second paragraph with a question (that then gets answered) carries forth the sense of befuddlement that researchers initially experienced and helps to convey why the discovery of HIV was a hugely important turning point. Using the self-glorifying Duesberg quote to launch the third paragraph makes the point about lingering skepticism while also introducing a portrait of a leading figure among the skeptics. While Set B is effective as well, Set C illustrates some of the more lively choices available to academic writers.

A last thing to note about topic sentences is that academic readers expect them to be at the beginning of the paragraph.⁵ That helps readers comprehend your argument. To see how, try this: find an academic piece (such as a textbook or scholarly article) that strikes you as well written and go through part of it reading just the first sentence of each paragraph. You should be able to easily follow the sequence of logic. When you’re writing for professors, it is especially effective to put your key sentences first because they usually convey your own original thinking, which, as you’ve read here, is exactly what your instructors are looking for in your work. It’s a very good sign when your paragraphs are typically composed of a telling key sentence followed by evidence and explanation.

Writing at Work

In the work world, people can be very busy and may not have time to read all the details of a proposal you might make. For this reason, in business writing, people want to know your point up front and then, if they want to look at your reasoning or support, they will read further. Just as in academic writing, in business writing, the reader should be able to read the first sentence of each paragraph and follow your logical thinking and reasoning.

Knowing this convention of academic writing can help you both read and write more effectively. When you’re reading a complicated academic piece for the first time, you might want to go through reading only the first sentence or two of each paragraph to get the overall outline of the argument. Then you can go back and read all of it with a clearer picture of how each of the details fit in.⁶ And when you’re writing, you may also find it useful to write the first sentence of each paragraph (instead of a topic-based outline) to map out a thorough argument before getting immersed in sentence-level wordsmithing. For example, compare these two scaffolds. Which one would launch you into a smoother drafting process?:⁷

Version A (Topic Outline):

I. Granovetter’s “Strength of weak ties”

A. Definition

B. Example—getting jobs

II. Creativity in social networks

A. Explanation

B. Richard Florida’s argument

III. Implications

A. For urban planners

B. For institutions of higher education

Version B (Key-Sentence Sketch):

The importance of networking for both career development and social change is well known. Granovetter (1973) explains that weak ties—that is, ties among acquaintances—are often more useful in job hunting because they connect job-seekers to a broader range of people and workplaces. …

Subsequent research in network analysis has shown that weak ties can promote creativity by bringing ideas together from different social realms. …

Richard Florida (2002) argues that cities would do well to facilitate weak ties in order to recruit members of the “creative class” and spur economic development. …

Florida’s argument can inspire a powerful new approach to strategic planning within colleges and universities as well. …

As you can see, emphasizing key sentences in both the process and product of academic writing is one way to ensure that your efforts stay focused on developing your argument and communicating your own original thinking in a clear, logical way.

Topic versus Controlling Idea

Topic sentences contain both a main idea (the subject, or topic that the writer is discussing) and a controlling idea (the writer’s specific stance on that subject). Just as a thesis statement includes an idea that controls a document’s focus, a topic sentence must also contain a controlling idea to direct the paragraph. Different writers may use the same main idea but can steer their paragraph in a number of different directions according to their stance on the subject. Read the following examples.

• Marijuana is a destructive influence on teens and causes long-term brain damage.
• The anti-nausea properties in marijuana are a lifeline for many cancer patients.
• Legalizing marijuana would create a higher demand for Class A and Class B drugs.

Although the topic—marijuana—is the same in all three topic sentences, the controlling idea differs depending on the writer’s viewpoint.

Exercise 1

Circle the topic and underline the controlling idea in each of the following topic sentences.

1. Exercising three times a week is the only way to maintain good physical health.
2. Sexism and racism are still rampant in today’s workplace.
3. Raising the legal driving age to twenty-one would decrease road traffic accidents.
4. Owning a business is the only way to achieve financial success.
5. Dog owners should be prohibited from taking their pets on public beaches.

Characteristics of a Good Topic Sentence

Five characteristics define a good topic sentence:

1. A good topic sentence provides an accurate indication of what will follow in the rest of the paragraph.

   Weak example. People rarely give firefighters the credit they deserve for such a physically and emotionally demanding job. (The paragraph is about a specific incident that involved firefighters; therefore, this topic sentence is too general.)

   Stronger example. During the October riots, Unit 3B went beyond the call of duty. (This topic sentence is more specific and indicates that the paragraph will contain information about a particular incident involving Unit 3B.)

2. A good topic sentence contains both a topic and a controlling idea or opinion.

   Weak example. In this paragraph, I am going to discuss the rising suicide rate among young professionals. (This topic sentence provides a main idea, but it does not present a controlling idea.)

   Stronger example. The rising suicide rate among young professionals is a cause for immediate concern. (This topic sentence presents the writer’s opinion on the subject of rising suicide rates among young professionals.)

3. A good topic sentence is clear and easy to follow.

   Weak example. In general, writing an essay, thesis, or other academic or nonacademic document is considerably easier and of much higher quality if you first construct an outline, of which there are many different types. (This topic sentence includes a topic and a controlling idea, but both are buried beneath the confusing sentence structure and unnecessary vocabulary. These obstacles make it difficult for the reader to follow.)

   Stronger example. Most forms of writing can be improved by first creating an outline. (This topic sentence cuts out unnecessary verbiage and simplifies the previous statement, making it easier for the reader to follow.)

4. A good topic sentence does not include supporting details.

   Weak example. Salaries should be capped in baseball for many reasons, most importantly so we don’t allow the same team to win year after year. (This topic sentence includes a supporting detail that should be included later in the paragraph to back up the main point.)

   Stronger example. Introducing a salary cap would improve the game of baseball for many reasons. (This topic sentence omits the additional supporting detail so that it can be expanded upon later in the paragraph.)

5. A good topic sentence engages the reader by using interesting vocabulary.

   Weak example. The military deserves better equipment. (This topic sentence includes a topic and a controlling idea, but the language is bland and unexciting.)

   Stronger example. The appalling lack of resources provided to the military is outrageous and requires our immediate attention. (This topic sentence reiterates the same idea and controlling thesis, but adjectives such as appalling and immediate better engage the reader. These words also indicate the writer’s tone.)

Exercise 2

Choose the most effective topic sentence from the following sentence pairs.

1. a. This paper will discuss the likelihood of the Democrats winning the next election.

   b. To boost their chances of winning the next election, the Democrats need to listen to public opinion.

2. a. The unrealistic demands of multi-national corporations are crippling workers' ability to achieve the American Dream.

   b. Multi-national corporations are crippling the economy because their workers are unable to balance work and personal lives.

3. a. Authors are losing money as a result of technological advances.

   b. The introduction of new technology will devastate the literary world.

4. a. Heavy metal is outdated and radio stations should stop playing it.

   b. Here is why radio stations should stop playing heavy metal.

Exercise 3

Using the tips on developing effective topic sentences in this section, create a topic sentence on each of the following subjects. Remember to include a controlling idea as well as a topic. Write your responses on your own sheet of paper.

1. An endangered species

   ____________________________________________

2. The cost of fuel

   ____________________________________________

3. The legal drinking age

   ____________________________________________

4. A controversial film or novel

   ____________________________________________

Writing at Work

When creating a workplace document, use the “top-down” approach—keep the topic sentence at the beginning of each paragraph so that readers immediately understand the gist of the message. This method saves busy colleagues precious time and effort trying to figure out the main points and relevant details.

Headings are another helpful tool. In a text-heavy document, break up each section with individual headings. These serve as useful navigation aids, enabling colleagues to skim through the document and locate paragraphs that are relevant to them.

Implied Topic Sentences

Some well-organized paragraphs do not contain a topic sentence at all. Instead of being directly stated, the main idea is implied in the content of the paragraph. Read the following example:

Heaving herself up the stairs, Luella had to pause for breath several times. She let out a wheeze as she sat down heavily in the wooden rocking chair. Tao approached her cautiously, as if she might crumble at the slightest touch. He studied her face, like parchment, stretched across the bones so finely he could almost see right through the skin to the decaying muscle underneath. Luella smiled a toothless grin.

Although no single sentence in this paragraph states the main idea, the entire paragraph focuses on one concept—that Luella is extremely old. The topic sentence is thus implied rather than stated. This technique is often used in descriptive or narrative writing. Implied topic sentences work well if the writer has a firm idea of what he or she intends to say in the paragraph and sticks to it. However, a paragraph loses its effectiveness if an implied topic sentence is too subtle or the writer loses focus.

Tip

Avoid using implied topic sentences in an informational document. Readers often lose patience if they are unable to quickly grasp what the writer is trying to say. The clearest and most efficient way to communicate in an informational document is to position the topic sentence at the beginning of the paragraph.

Exercise 4

Identify the topic sentence, supporting sentences, and concluding sentence in the following paragraph.

The desert provides a harsh environment in which few mammals are able to adapt. Of these hardy creatures, the kangaroo rat is possibly the most fascinating. Able to live in some of the most arid parts of the southwest, the kangaroo rat neither sweats nor pants to keep cool. Its specialized kidneys enable it to survive on a miniscule amount of water. Unlike other desert creatures, the kangaroo rat does not store water in its body but instead is able to convert the dry seeds it eats into moisture. Its ability to adapt to such a hostile environment makes the kangaroo rat a truly amazing creature.

Collaboration

Please share with a classmate and compare your answers.

Contributors and Attributions

• Adapted from Writing for Success. Provided by: The Saylor Foundation. License: CC-NC-SA 3.0.
• Adapted from Writing in College. Authored by: Amy Guptill. Provided by: Milne Publishing. License: CC-NC-SA 4.0

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• "How to Write a Topic Sentence." Authored by: Scribbr.  License: All Rights Reserved. License Terms: Standard YouTube license.

This page most recently updated on June 23, 2020.