Ways of targeting vocabulary words have been evolving in the past decade, Mr. Shanahan said. Reading materials developed in the early 1990s tend to focus on the phonics of words, so the word "cat" might have been chosen to highlight the "-at" sound, rather than because educators need to teach children what the word means.
Mr. Shanahan said he believes the lack of vocabulary cohesion comes from the attempt to choose more difficult words from upcoming texts.
"It gets very tricky," he said. "The words aren't terribly important; they aren't words you'd really care if the children know or not. If the next story has a platypus in it, that's a hard word; we might as well teach it. ... We've managed to get publishers off 'cat,' but they've swung over to 'platypus.' "
I found this to be a particularly clear and informative article. Like most of the conversation swirling around all things Common Core, it is more appropriately aimed at textbook (etc) publishers than classroom teachers, and like most highly specific literacy research it is aimed at the early elementary grades. If you use the same approach to high school, I don't think it makes as much sense, nor is it as well backed by research.