US gym membershipsUS self-published books per year
The American who writes a book and the American who joins a gym have, since 2015, been multiplying at nearly the same rate (r = 0.959), which is either an endorsement of self-improvement or a warning about oversupply. One produces a manuscript; one produces a membership card. Both tend, in the long run, to be half-finished.
US self-published book output, measured via ISBN assignments, rose from roughly 700,000 titles per year in 2015 to over 2 million by 2023, riding Amazon KDP and the near-zero marginal cost of uploading a Word document to a printer; US gym memberships climbed from about 55 million to over 66 million in the same window, led by Planet Fitness's $10/month model and the boutique boom of Barry's, Equinox, and Soulcycle. Both are expressions of the same self-improvement-as-identity wave, amplified by social media's pressure to produce visible progress. The average completion rate on a self-published book is around 40%; the average year-over-year gym membership retention is about 57%; both are better than you'd think and worse than the owner intended.
The manuscript grows. The deadlift grows. Both plateau eventually, yet the subscription continues.
As an Amazon Associate, getspurious.com earns from qualifying purchases. Learn more.
Want to learn more about why correlations like “US gym memberships” vs “US self-published books per year” don't prove causation? Read our guide to statistical thinking.
