Letters mailed by the US Postal Service

As Americans mailed fewer letters from 2010 to 2021, they self-published dramatically more books, achieving a -0.97 correlation that implies the entire self-publishing industry is running on redirected postal energy. USPS first-class mail volume fell from around 80 billion pieces in 2010 to under 30 billion by 2021 as email and messaging finished their work. Self-published titles rose from 133,000 to over 800,000. Apparently the urge to communicate is conserved, it simply changes medium.

Both trends are driven by digital disruption of traditional communication and distribution channels. Email, messaging apps, and electronic billing devastated first-class mail volumes throughout the 2010s, with USPS letter volumes declining by roughly 50% over the decade. Simultaneously, digital publishing platforms — particularly Amazon KDP and Smashwords — eliminated the traditional barriers to book publication, causing self-published title counts to grow tenfold. Both are manifestations of the same disintermediation dynamic: digital technology removing institutional gatekeepers from communication, whether that communication is a letter or a novel.

Some spurious correlations are less spurious than they appear. Here, both curves are driven by the same underlying force — digital disruption — and the inverse relationship reflects a genuine substitution of one medium for another, even if the specific link is indirect.