Subpopulation Analysis. As shown in (1), bounds can be obtained by conditioning on covariates, X. To explore the possibility of heterogeneity in the ATE across different subpopulations, we condition on time period and/or OECD status. Tables 2 and 3 condition on time period, with Table 2 presenting bounds for the pre-WTO period, 1960-1994, and Table 3 presenting the corresponding bounds for the WTO period, 1995-2001. The corresponding graphs for the MTS and MTS-MIV bounds are shown in Figures A1-A3 in the Appendix. Note, Panel III is missing in Table 3 due to a lack of variation in the outcome in the WTO period. That said, the results for the bounds for the two outcomes in Panels I and II are not qualitatively different across the periods representing a change from the GATT to the WTO. Moreover, comparing Panel III in Tables 1 and 2 (i.e., the full sample versus only the pre-WTO period) also does not indicate any substantive changes in the bounds. Tables 4, along with the left panel of Figures A4-A6 in the Appendix, provide separate bounds for non-OECD countries over the entire sample period.17 While the bounds obtained under exogenous selection as well as no selection assumptions are largely similar across Tables 1 and 4, the MTS-MIV bounds differ to an extent. More precisely, the upper bounds corresponding to new participation (Panels I and II) are smaller for non-OECD countries. Under the assumption of no misclassification, the MTS-MIV bounds for non-OECD countries are [−0.331, 0.036] and [−0.306, 0.028] in Panels I and II (Table 4), respectively. For the full sample, the corresponding bounds are [−0.398, 0.163] and [−0.366, 0.191]. Thus, while the ATE cannot be signed for either group, we can rule out the possibility of a substantial increase in the propensity of non-OECD countries to enter into new MEAs resulting from accession into the WTO. This is a meaningful result, obtained under relatively minor assumptions, as it implies that integrating lesser developed countries into the global economy will do little to facilitate cooperation on international environmental issues.18 Table 5 and Figures 4 and 5 display the final set of results for non-OECD countries in the WTO period, 1995- 2001.19 Only Panels I and II are included as there is insufficient variation in the data when using the third measure of MEA participation. Here, we obtain a xxxxx finding. Under the assumption of no misclassification, the MTS-MIV bounds are now strictly negative in both panels; they are strictly n...
