Innovations spread sideways through a "hummingbird effect"

Johnson's central organizing idea is what he calls the hummingbird effect, borrowed from the way flowering plants and hummingbirds evolved together in ways neither could have anticipated: an innovation designed to solve one problem ends up, through unplanned side effects, transforming a completely unrelated field. Gutenberg's printing press was built to mass-produce Bibles, but the resulting explosion of reading exposed how many people had undiagnosed farsightedness, which created sudden commercial demand for corrective lenses, which in turn built the glass-grinding expertise that later produced telescopes and microscopes.

This pattern recurs throughout the book: air conditioning, invented to solve industrial humidity problems, eventually reshaped where and how Americans could live, migrate, and even vote, by making the hot Sun Belt habitable year-round. Johnson's point is that these effects are essentially impossible to predict in advance, which argues against both hero-worship of individual inventors and confident forecasting about where any single innovation will ultimately lead.

Takeaway: the biggest downstream effects of an invention are rarely the ones its inventor intended or foresaw.