Google cooperated and built the software for making graphs open to the public. The initial version of Google’s cultural exploration site began at the end of 2010, based on more than five million books, dating from 1500. By now, Google has scanned 20 million books, and the site is used 50 times a minute. For example, type in “women” in comparison to “men,” and you see that for centuries the number of references to men dwarfed those for women. The crossover came in 1985, with women ahead ever since.

In work published in Science magazine in 2011, Mr. Michel and the research team tapped the Google Books data to find how quickly the past fades from books. For instance, references to “1880,” which peaked in that year, fell to half by 1912, a lag of 32 years. By contrast, “1973” declined to half its peak by 1983, only 10 years later. “We are forgetting our past faster with each passing year,” the authors wrote.

JON KLEINBERG, a computer scientist at Cornell, and a group of researchers approached collective memory from a very different perspective.

Their work, published last year, focused on what makes spoken lines in movies memorable. Sentences that endure in the public mind are evolutionary success stories, Mr. Kleinberg says, comparing “the fitness of language and the fitness of organisms.”

As a yardstick, the researchers used the “memorable quotes” selected from the popular Internet Movie Database, or IMDb, and the number of times that a particular movie line appears on the Web. Then they compared the memorable lines to the complete scripts of the movies in which they appeared — about 1,000 movies.

To train their statistical algorithms on common sentence structure, word order and most widely used words, they fed their computers a huge archive of articles from news wires. The memorable lines consisted of surprising words embedded in sentences of ordinary structure. “We can think of memorable quotes as consisting of unusual word choices built on a scaffolding of common part-of-speech patterns,” their study said.

Consider the line “You had me at hello,” from the movie “Jerry Maguire.” It is, Mr. Kleinberg notes, basically the same sequence of parts of speech as the quotidian “I met him in Boston.” Or consider this line from “Apocalypse Now”: “I love the smell of napalm in the morning.” Only one word separates that utterance from this: “I love the smell of coffee in the morning.”