A couple of books I’ve read lately made me think about themes or plot devices that used to be common in mystery novels, but no longer work for “contemporary” mysteries—those set in the 21st century. *There may be “spoilers” in the following material; proceed at your own risk*
Two of these are significant plot points in a Perry Mason mystery I just finished, *The Case of the Blond Bonanza* (1962). A significant plot point involves a man who disappeared when a boat overturned 14 years before the events in the book; he then re-appears with a different name and a new life. This is a fairly common device in some books, either in the plot or (as in *The Maltese Falcon* [the Flitcraft parable (for a lengthy discussion: http://j-nelson.net/2015/01/twenty-writers-dashiell-hammett-the-flitcraft-parable-from-the-maltese-falcon/)] a narrative device. (A recent example is in Lawrence Block’s novella, *Resume Speed.*) In a time before nearly everyone had a Social Security number, and getting employment did not require you to have a SSN, drifting from place-to-place with a very malleable identity more-or-less works as a plot device. But in a “contemporary” story, it’s something that almost demands explanation—how does the character get away with it?
The second plot point in the Gardner book involves a PI keeping one of the characters under surveillance. He has to leave his surveillance position to make a phone call, which eventually tips Perry off as to the actual course of events. (Again, this is a common bit in a lot of PI novels—the need to find a phone to report in/ask for help/etc.) But now, with cell phones, that no longer works.
The other plot device comes mostly from “caper” books, especially those involving the heist of a company payroll. [Donald Westlake as Richard Stark, *The Score* (1964) is a good example.] As late as the mid-1960s, this seems somewhat plausible. But by about the late 1960s/early 1970s (and in my own case, from the very first summer job I had, working for a grocery store chain), companies generally paid by check, even to employees without bank accounts, and today most large firms pay only by direct deposit. So another plot device that worked fairly recently would probably raise eyebrows in a book set in the US in the 2010s.