I think 80-81 was when USA signed the Bern convention that defined copyright as life+50 for authors (when assigned to corporations it has a fixed duration of, iirc, 90 years).
But the Bern convention also have a stipulation that all signatories have to respect the duration of the initial nation of publication, and that can be longer than the minimum terms of the convention agreement.
Thus you get a ratcheting effect where multinationals will try to convince national governments to up their copyright terms to be "more competitive".
A kind of inverse to the race to the bottom that they first did on taxes between US states (leading to Delaware being the state to file your incorporation in), and has since applied across the globe under the banner of competition.
BTW, there is a claim that Lord of the Rings became popular because a US publisher thought he didn't need to respect Tolkien's UK copyright when publishing a cheap paperback. At the time USA had not yet signed the Bern convention.
Frankly it seems like a historic pattern where a industrial nation will begin to slow down, try to shore up its economy by using IP laws, and another nation coming along and ignoring those laws to bootstrap their own industry, and then repeating the patterns some decades down the road.
So far the changeover has been UK to USA to China. And you can basically see China trying to clamp down on their lax IP adherence right now.
