It takes heroic amounts of practice to boost digit span, mostly by using chunking procedures (which themselves have to be learned) so as to improve the encoding of random number strings. There are better things to do in life. Digit span forwards is a measure of the length of acoustic memory trace that can be reported on before it fades. If you have the misfortune to number your digits in Welsh, then because the names of digits in that language are long, you will be able to fit in slightly fewer digits into the quickly fading memory trace. Digits backwards, of course, can be affected by the same variables, yet it is the additional difficulty of that task which is particularly interesting, and not strictly relevant to these discussions about the increase in digits forwards since 1923.
However, it remains plausible that practising numbers in real life, like remembering telephone numbers, might have an impact on digit span forwards. Reader Richard Harper kindly sent me the Ngram for “combination locks”. Two can play at that game. Undaunted, I have immediately retaliated by adding in “telephone number” and “password” as comparable phrases, with the results shown below:
This shows that “combination lock” is, relatively speaking, nowhere. “Telephone number” became frequent in 1940 but after a steep rise began to fall by the millennium. Mobile phones allow names to be substituted for numbers, reducing memory load. The surprise is that “password” had been popular from the 1900s onwards, but the demanding, demeaning, insolent, insistent word booms after 1980, and becomes virally toxic just after 2000. Will no-one spare us from this pest? Mercifully, the agony is somewhat abating. Perhaps “iris” or that old standby “fingerprint” is responsible for saving us from the need to remember which unbreakable code we used to protect our bank accounts, payment systems, gas bills, electricity bills, mobile phone bills, google accounts, airline memberships, newspaper subscriptions, publisher’s login details, online supermarket accounts, amazon accounts, pension payment accounts, department store accounts, linkedin accounts, ISP accounts, anti-virus accounts, paypal, researchgate, ucl library accounts, science direct accounts, ebay accounts, squirrelmail, consumer organisations, and even places where you have once bought one random item, before sitting down to have a coffee near a wifi that will not let you in without a password, but will reject you once you give your email, because it is already taken by your former self, a slightly younger version who generated a password with insouciance, and never thought to write it down.
Personally, I doubt the demands of modernity have done much for digit span forwards. It is rarely a matter of life or death so the six or so generations since the rise of telephony will not confer any selective advantage. Gains may be due to general improvements in health and living standards. As regards passwords, I suppose it might be worth testing if alphanumeric recall had improved considerably between 1980 and 2000. However, I can think of a reason why recalling passwords will not have boosted any memory ability. A large section of the population, when asked to generate a password, obligingly choose “password”. Folk who resemble them in wisdom increase complexity by listing single digits in order of magnitude, no mean feat. Perhaps all these good people are very trusting, or cannot calculate probabilities, or simply have nothing to lose. A final word: digits forwards is only predictive when scores are low: high scores are less indicative of intellect. Rest easy.