Main Ideas, Topic Sentences, and Supporting Details

Football is not a healthy activity for kids. It causes head injuries that harm the ability to learn and achieve. It causes painful bodily injuries that can linger into adulthood. It teaches aggressive behavioral habits that make life harder for players after they have left the field.

The topic of this paragraph is youth football, and the main idea is that kids should not play the game. Note that if you are asked to state the main idea of a persuasive text, it is your job to be objective. This means you should describe the author’s opinion, not make an argument of your own in response.

Both of the example paragraphs above state their main idea explicitly. Some texts have an implicit, or suggested, main idea. In this case, you need to figure out the main idea using the details as clues.

FOR EXAMPLE
The following fictional paragraph has an implicit main idea:

Daisy parked her car and sat gripping the wheel, not getting out. A few steps to the door. A couple of knocks. She could give him the news in two words. She’d already decided what she was going to do, so it didn’t matter what he said, not really. Still, she couldn’t make her feet carry her to the door.

The main idea here is that Daisy feels reluctant to speak to someone. This point is not stated outright, but it is clear from the details of Daisy’s thoughts and actions.

Topic Sentences

Many paragraphs identify the topic and main idea in a single sentence. This is called a topic sentence, and it often appears at the beginning of a paragraph. However, a writer may choose to place a topic sentence anywhere in the text.

Some paragraphs contain an introductory sentence to grab the reader’s attention before clearly stating the topic. A paragraph may begin by asking a rhetorical question, presenting a striking idea, or showing why the topic is important. When authors use this strategy, the topic sentence usually comes second:

Have you ever wondered how your body fights off a nasty cold? It uses a complex defense mechanism called the immune system. The immune system fights off harmful bacteria, viruses, and substances that attack the body. To do this, it uses cells, tissues, and organs that work together to resist invasion.

Here, the first sentence grabs the attention, and the second, boldfaced topic sentence states the main idea. The remaining sentences provide further information, explaining what the immune system does and identifying its basic components.

COMPARE!
The informational paragraph above contains a question that grabs the attention at the beginning. The writer could convey the same information with a little less flair by omitting this device. The version you read in Section 1 does exactly this. (The topic sentence below is boldfaced.)

The immune system is the body’s defense mechanism. It fights off harmful bacteria, viruses, and substances that attack the body. To do this, it uses cells, tissues, and organs that work together to resist invasion.

Look back at the football paragraph from Section 1. Which sentence is the topic sentence?

Sometimes writers wait until the end of a paragraph to reveal the main idea in a topic sentence. When you’re reading a paragraph that is organized this way, you may feel like you’re reading a bit of a puzzle. It’s not fully clear what the piece is about until you get to the end:

It causes head injuries that harm the ability to learn and achieve. It causes painful bodily injuries that can linger through the passage of years. It teaches aggressive behavioral habits that make life harder for players after they have left the field. Football is not a healthy activity for kids.

Note that the topic—football—is not actually named until the final, boldfaced topic sentence. This is a strong hint that this final sentence is the topic sentence. Other paragraphs with this structure may contain several examples or related ideas and then tie them together with a summary statement near the end.

Supporting Details

The supporting details of a text develop the main idea, contribute further information, or provide examples.

All of the supporting details in a text must relate back to the main idea. In a text that sets out to define and describe the immune system, the supporting details could explain how the immune system works, define parts of the immune system, and so on.

Main Idea: The immune system is the body’s defense mechanism.

Supporting Detail: It fights off harmful bacteria, viruses, and substances that attack the body.

Supporting Detail: To do this, it uses cells, tissues, and organs that work together to resist invasion.

The above text could go on to describe white blood cells, which are a vital part of the body’s defense system against disease. However, the supporting details in such a text should not drift off into descriptions of parts of the body that make no contribution to immune response.

Supporting details may be facts or opinions. A single text can combine both facts and opinions
to develop a single main idea.

Main Idea: Football is not a healthy activity for kids.

Supporting Detail: It teaches aggressive behavioral habits that make life harder for players after they have left the field.

Supporting Detail: In a study of teenage football players by Dr. Sophia Ortega at Harvard University, 28% reported involvement in fights or other violent incidents, compared with 19% of teenage boys who were not involved in sports.

The first supporting detail above states an opinion. The second is still related to the main idea, but it provides factual information to back up the opinion. Further development of this paragraph could contain other types of facts, including information about football injuries and anecdotes about real players who got hurt playing the game.

Let’s Review!

• The main idea is the most important piece of information in a text.
• The main idea is often expressed in a topic sentence.
• Supporting details develop the main idea, contribute further information, or provide examples.