Give some stream of consciousness and then the answer. Supply-side economics is a macroeconomic theory arguing that economic growth can be most effectively created by lowering taxes and decreasing regulation. According to supply-side economics, consumers will then benefit from a greater supply of goods and services at lower prices and employment will increase. It was started by economist Robert Mundell during the Ronald Reagan administration.

Based on this passage, when did supply side economic policies begin to grow in popularity?
The relevant information to answer the above question is: It was started by economist Robert Mundell during the Ronald Reagan administration.
Therefore, the answer is during the Ronald Reagan administration.