Lawyers seldom design topic sentences deliberately; yet, explicit initial topic sentences demonstrably improve comprehension of difficult material through the cognitive mechanism of semantic priming, whereby concepts become more accessible after being activated when the reader entertains related concepts. Instead of using topic sentences, lawyers often avoid writing them by using trivial statements of dates and case names as substitutes (see http://tinyurl.com/rx3bth), since composing and revising topic sentences seems dreary work. (See, for example, http://tinyurl.com/qw2kzl.)
Most of the discussion of topic sentences — which occurs in the primary-education literature and in the deliberations of teachers of freshman composition — exaggerates the generality of topic-sentence usage. Without the aid of research, educators have long extolled the topic sentence as prerequisite for a proper paragraph. "Language Arts" instruction in the early grades goes further than recommending a topic sentence for every paragraph, calling for a "summary sentence" at each paragraph's end.
Students inevitably notice that, except in textbooks, paragraphs aren't nearly so regular, including paragraphs constructed by the best writers. Even when paragraphs contain strong topic sentences, some serve better at the paragraph's conclusion or, preceded by transitional sentences, toward the paragraph's middle. Students conclude that their searches for topic sentences in English classes serve as an exercise rather than a tool for paragraph construction; that teachers don't criticize the students' schoolday paragraphs for lacking topic sentences reinforces this conclusion. Like any exercise, the construction of topic sentences became a dreary business, and going beyond performing such exercises becomes a mark of the students' sophistication, of their adulthood as writers.
Teaching students a distorted view of paragraph construction is bound to cause disillusionment; so, writers must fashion a more nuanced view of topic-sentence usage. The distortion became apparent when the education world was rocked by Braddock's 1974 study, indicating that initial topic sentences rarely occur in the paragraphs of professional writers. Later research qualified Braddock's findings by showing that topic-sentence usage among professional writers differs with the kind of writing. Researchers found that initial sentences vary in their closeness to the educators' idealization and are classifiable into two broad types: natural topic sentences and ideal topic sentences. Natural topic sentences lack some of the characteristics of classic topics; they serve as point sentences instead of tertiary thesis statements. Ideal topic sentences are those still taught in the schools; each states a claim supported by the rest of the paragraph. Ideal topic sentences grow more useful with the material's difficulty.
An ideal topic sentence doesn't best serve every paragraph. Sometimes an explicit topic sentence will be too heavy-handed if a measure of subtlety is called for; sometimes a paragraph will already be so cohesive that inserting an ideal topic sentence detracts from the paragraph's effectiveness; sometimes a topic is better placed somewhere besides the initial sentence. Despite their lack of universal application, topic sentences are particularly important in writing legal briefs, where unnecessary subtlety is misplaced. Using ideal topic sentences sharpens and polishes a brief dealing with complicated substantive law. On matters where the judge can be presumed knowledgeable, natural topic sentences may avoid the appearance of condescension, but natural topic sentences still require revision — often, reorganization of the sentences — so they correspond to their paragraph's content.