fall under WTO discipline, the restricted policy choice set induces more substitution of import protection toward TTB policies. The estimated IRR of 1.04 on this variable in specification (7) is larger than 1 as expected, though it is not statistically significant. Second, we also introduce a TTB “stock” variable defined as the count of HS-06 products over which the policy-imposing economy _j_ already had a TTB in effect against trading partner _i_ in _t-1_ . We expect an IRR that is less than 1 for this variable; i.e., the more imported products already subject to a TTB, the less scope for the economy to impose new TTBs. The estimated IRR is 0.99 and statistically significant.<sup>20</sup> The downside to including these two variables is that it restricts our sample to the post-1995 period, further limiting the business cycle variation in the data.<sup>21</sup> Therefore, to understand how our results are affected by omitting these controls for the available policy choice set, specification (8) reestimates the baseline model on the smaller sample of data used in specification (7). A comparison of specification (7) to (8) reveals very little change to the IRRs for the key macroeconomic determinants. We conclude that omitting controls for the trade policy choice set does not appear to affect the results for these policy-imposing economies over this time period.

To summarize Table 2, we present evidence from 1988:Q1-2008:Q3 that macroeconomic shocks impact import protection through temporary trade barriers.<sup>22</sup> First, there is strong evidence of a countercyclical relationship between negative shocks to domestic employment (and to a lesser extent, domestic real GDP growth) and new import protection. Second, our comprehensive temporary lack of inter-temporal variation, relative to all of the other variables included in the estimation which are defined at the _quarterly_ frequency, in addition to the lack of trading-partner variation, is likely a contributing explanation to the statistically insignificant IRR estimate described in the text.

20 In unreported results available from the authors, we have also estimated versions of the model in which we introduce each of these two variables separately, and the broad pattern of results does not change.

21 First, the required data on WTO tariff bindings is only relevant and available for the post-1995 period. Second, the TTB stock variable is only constructed for TTBs imposed since the HS system has been in effect beginning in 1988. The implication is that earlier in our sample, the stock measure based on available HS data would severely underestimate the actual TTBs in effect as it is not able to consider all of the TTBs imposed prior to 1988 that were still effect in the early 1990s. This is important for the historical users of TTBs such as Australia, Canada, European Union, and the United States. Thus the TTB stock measure constructed from HS-06 data becomes more accurate over our sample period as (unaccounted for) TTBs that governments imposed prior to 1988 are increasingly removed.

> 22 A related research question is whether government decisions on TTB _removals_ are symmetrically affected by macroeconomic conditions. For example, when a government examines its previously-imposed antidumping import restrictions under the WTO’s guidelines for Sunset Reviews, is it less likely to remove such a measure if the domestic economy is undergoing a period of weak growth or a high rate of unemployment? This is an important and unaddressed question that we leave for future research, as it is beyond the scope of the current analysis for data limitation reasons.

15