[gov2001-l] Posttreatment bias and matching

For our paper, Amy and I have a binary treatment variable that indicates an increase in the diversity index of at least 10% from 1970 to 2000. What we'll be testing via matching will be the influence of "treatment" (rapid increase in diversity) on growth from 1970 to 2000 (dep. var). Should I only balance on conditions prior to 1969 to avoid posttreatment bias since the treatment is growth in diversity from 1970 to 2000? Or do we want to balance on everything (even conditions in the middle of this time period) and only worry about post treatment bias in the regressions we run once the treatment and control group have been made? Thanks! Suzanna