  Research underscores the role of naturalization in enhancing immigrants'
socio-economic integration, yet application rates remain low. We estimate a
policy rule for a letter-based information campaign encouraging newly eligible
immigrants in Zurich, Switzerland, to naturalize. The policy rule assigns one
out of three treatment letters to each individual, based on their observed
characteristics. We field the policy rule to one-half of 1,717 immigrants,
while sending random treatment letters to the other half. Despite only moderate
treatment effect heterogeneity, the policy tree yields a larger, albeit
insignificant, increase in application rates compared to assigning the same
letter to everyone.
