Historians of technology have long puzzled over why some inventions diffuse rapidly across an economy while others, of comparable engineering merit, languish for decades before widespread adoption. One influential account attributes the gap to complementary investment: a new technology often delivers its full value only once firms have made costly, technology-specific adjustments to organization, worker training, and supporting infrastructure, and firms delay these adjustments until they are confident the technology will not soon be superseded. On this account, slow diffusion reflects rational hesitation rather than ignorance or irrationality on the part of adopters.

A rival account emphasizes the incumbents whose existing investments the new technology threatens to strand. Firms and workers with capital, whether physical or human, sunk into the prevailing technology have a direct financial stake in delaying a successor that would devalue those sunk investments, and where such incumbents hold political or market power, they may act — through lobbying, standard-setting, or exclusive contracts — to slow adoption beyond what complementary-investment considerations alone would predict.

The two accounts are not mutually exclusive, and most documented cases of slow diffusion likely reflect some combination of both mechanisms; the accounts diverge sharply, however, in their policy implications. If hesitation is rational and complementary investment is the binding constraint, subsidizing the up-front cost of that investment should accelerate adoption. If incumbent resistance is doing most of the work, however, such subsidies may simply be captured by the very incumbents seeking to delay the technology, leaving diffusion largely unchanged.