An expository essay, often required in high school and college classes, allows you to explore an opinion or make an argument about a particular idea. The thesis statement usually appears at the end of the introductory paragraph of your essay, making a clear declaration about the opinion you wish to defend. The rest of your paper should speak back to this sentence and should be supported by reasons and evidence, such as statistics, facts, professional opinions and analyses.
Contestable and Debatable
A thesis statement should be contestable and debatable, which means that others could reasonably object to it. For example, it's unlikely that you will find sane objectors to the statement "murder is bad for society" unless that statement is qualified in some way (for example, if you are against the death penalty -- and then your statement will need to be more specific). Likewise, a thesis statement cannot be a fact, such as "the earth travels around the sun." This is a widely held belief, and such a paper would offer only an explanation, not an argument.
A thesis statement should also be supportable. If you can't provide support for your statement, there won't be much content in the body of your paper. An unsupportable statement could be one attributed solely to taste -- for example, "chocolate is the best ice cream flavor" -- or one based only on your belief system, such as "the soul lives on after death." Make sure to qualify your statement correctly, because all-or-nothing terms, such as always and never, are often more difficult to support.
A simple or vague claim often leads to an unfocused paper, while a rich, complex and specific claim leads to a coherent paper. For example, the thesis "college athletics are unfair" will lead to a very desultory paper. A specific statement about why they are unfair -- "college athletics lead to an unfair distribution of scholarship funds" -- will result in a clearer paper. Depending on the kind of paper you want to write, you might be even more specific by offering a reason you will expound upon in your paper in more detail.
Sometimes, it is appropriate to frame your paper as a solution to a problem. "Alcohol abuse in college is out of control" is too vague, while "alcohol abuse in college can be limited by setting stricter on-campus policies" confronts a similar topic in a more specific way. Of course, being too specific can also get you into trouble. The specificity of your thesis should be appropriate to the length of your paper -- you want to be able to adequately cover your subject and prove your point in the required number of pages.
Finally, consider the wording of your thesis. While your statement should be conceptually complex, it shouldn't have wording that is too complicated. Your statement should be clear, concise and grammatically correct. Have a friend or peer read it over and offer feedback. Write it and rewrite it. Make sure it says what you mean it to say.