I spotted some tweets this morning about increases in the sales of physical media, including books.

These developments seem to overturn the narrative of ever-greater cannibalisation by online consumption.

But that narrative assumes physical and digital versions are always substitutes for each other, when there is no a priori reason to think that substitutability is boundless. The same substitution assumption was made about telephones when they first spread, but the empirical evidence seems to be that telephone and face-to-face contact might have become complements for each other rather than substitutes. The easier it became to arrange to meet someone in person, the more often it happened.

This is unlikely to be true for all physical media. While it is easy to see how reading part of a book online, or an article about a book, could start to increase physical sales, or how listening to music online can stimulate concert-going and maybe CD buying (even LPs!), it is less obviously true about newspapers, say. I don’t want to be all Pollyanna about this – and Anita Elberse’s [amazon_link id=”0805094334″ target=”_blank” ]Blockbusters[/amazon_link] for one provides some of the gloomier evidence about the effect of the switch to digital on most content suppliers even if the aggregate figures look healthy.

But the moral is, at least be aware of what you assume about substitutes and complements when predicting the future impact of digital; if you haven’t thought about it, you’ve still made an assumption. It might be wrong.

[amazon_image id=”0571309224″ link=”true” target=”_blank” size=”medium” ]Blockbusters: Why Big Hits – and Big Risks – are the Future of the Entertainment Business[/amazon_image]