Joseph M. Williams, my favorite writing authority, advises:
Some argue that the harder we have to work to understand what we read, the more deeply we think and the better we understand. Everyone should be happy to know that no evidence supports so foolish a claim, and substantial evidence contradicts it. (Williams, "Style: Lessons in Clarity and Grace" (9th ed. 2006) p. 221.) [HT: Vlastimil Vohánka, Comment to Cognitive Fluency: Simpler Isn’t Always Better.]
Williams’s summation is outdated after researchers reported that difficult texts promote better learning. (See Diemand-Yauman, C., et al., "Fortune favors the bold (and the italicized): Effects of disﬂuency on educational outcomes." (2010) Cognition.) Williams errs because he confuses Clarity, which measures the likelihood that the message succeeds, with fluency, which measures the effort the reader must expend to receive the message.
Social-psychological research points to two reasons for the divergence between Clarity and cognitive fluency (also called simplicity or cognitive ease). First, some matters are more intelligible and some beliefs more malleable when they engage an abstract (or “far”) construal level rather than a concrete (or “near”) construal level. Then, disfluent items will be more comprehensible. (“Construal-level theory: Matching linguistic register to the case's granularity,” in this blog, and “A taxonomy of political ideologies based on construal-level theory,” in Juridical Coherence, discuss construal-level theory.)
Second and more important practically, a trade-off between fluency and cohesion should discourage writers from always opting for the most fluent sentences, which must be bought at the expense of cohesion. The second reason explains disfluent clarity in legal writing better than the first because the writer should approach messages calling for abstract construal amelioratively, by imposing greater cohesion or greater Concision to complexify the text.
Joseph M. Williams himself best pinpoints the tension between fluency and cohesion.
The problem—and the challenge—of English prose is that, with every sentence we write, we have to strike the best compromise between the principles of local clarity and directness and the principles of cohesion that fuse separate sentences [ideas] into a whole discourse. But in that compromise, we must give priority to those features of style that make our discourse seem cohesive, those features that help the reader organize separate sentences [ideas] into a single, unified whole. (Williams, Style: Towards Clarity and Grace (1995) p. 48 [emphasis added].)
My change—“ideas” for “sentences”—emphasizes that consolidating the information contained in several sentences into a single sentence can contribute to cohesion.
The diagram below depicts the interactions leading from their causes to the two substantive Writing Virtues. (I save for another day the interaction between cohesion and omission, causing Concision).
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The diagram shows that writers must balance fluency and cohesion for Clarity; omission and cohesion for Concision. The centrality of cohesion, which contributes to both Clarity and Concision, helps explain why Williams is correct to stress cohesion over fluency (“local clarity”).
Confusion of fluency with Clarity—or at least, emphasis of fluency over cohesion—is the main technical deficiency promoted by the “plain writing” school.