New Delhi, Oct. 5 -- One of the most famous and widely studied natural experiments in modern history is the division of North Korea and South Korea at the end of World War II. A natural experiment occurs when a naturally occurring event creates a situation where similar groups are exposed to different conditions, letting policymakers study the outcomes.

In economic prosperity, life expectancy and much else, South Korea has had vastly different outcomes than the North, even though they were once a single nation with a shared language, culture, geography and ethnic background.

A broadly similar thing is underway in three Indian states that held legislative elections over the last couple of years-Odisha, West Bengal and Tamil Nadu. In all ...