Difference-in-difference strategy: requires two differences l Difference 1: Tea planting areas have higher relative wages for women l Women have a comparative advantage in cultivating tea. l Why cant we compare areas that plant tea with those that do not plant tea? l Difference 2: Value of planting tea rose over time due to post-Mao reforms l Why cant we just look at what happens pre-post reforms?

formula: The DID estimate of the impact of high female relative wage on the male-to-female ratio: (b-a) (d-c). Parallel trends assumption: had the tea-planting areas been growing cereals, the change in the outcome variable in the tea planting areas would be (d-c).