Physics
The Diffusion of Direct Democracy Instruments Across Established Democracies
Quick fact
Although Switzerland pioneered direct democracy in the 19th century, most established democracies only adopted these instruments in waves during the 20th century, often after political crises or constitutional reforms.
Why this is interesting
You've probably voted in elections, but in some countries you can also vote directly on laws. Why do some democracies let citizens decide policies themselves, while others don't?