vironments, on the other hand, subject to the Limão (2006) and Karacaovali and Limão (2008) studies, resulted in preference margins that were much smaller with potentially less scope for trade diversion. Second, the US and EU preferences were more unilateral in nature. The theory in Limão (2007) emphasizes the non-reciprocal nature of US and EU preferences and that they were o¤ered as compensation for countries that took up non-trade obligations in areas such as environmental or labor standards, intellectual property rights protection, and supporting the war on drugs. An open question for research is whether building block e¤ects may be more likely to dominate in reciprocal PTAs.<sup>105</sup> The rising importance of “WTO-extra”provisions in PTAs (Horn, Mavroidis, and Sapir, 2010) suggests that this should be a priority area for additional research.

Preferential tari¤s and MFN tari¤s are certainly not the end of the line when it comes to trade policy, as there are a number of other potential non-tari¤ barriers to trade. The GATT/WTO provides several exceptions that countries can invoke to implement higher levels of protection for legitimate environmental, health, or other public safety concerns, for example under Article XX. Furthermore, most of the same major economies involved in multilateral and preferential trade liberalization since the late 1980s are also major users of antidumping and safeguards (Bown, 2011a), another major category of GATT/WTO exceptions allowing countries to temporarily implement higher levels of import protection under certain conditions.<sup>106</sup> While the general relationship between PTA liberalization and non-tari¤ barriers use is not yet well understood, here we highlight a recent paper that initiated the investigation of this relationship.

Antidumping is the most frequently applied TTB (temporary trade barrier) policy in use across countries and over time since the 1980s. Prusa and Teh (2010) use a cross-country sample involving 80 PTAs and antidumping use dating back to 1980. While there is only a modest impact of PTA formation on the overall use of antidumping, after controlling for other aggregate-level determinants, there is evidence of important di¤erences in policy treatment between PTA partners and nonpartners. PTA implementation is associated with both a reduction in antidumping actions against new PTA partners and an increase against non-partners. Prusa and Teh attribute some of this post-PTA change in behavior to PTA variation in the legal provisions a¤ecting antidumping use - i.e., the sort of “WTO-plus”provisions described in more detail in Section 2.2. In any event, these ...ndings suggest that reorientation of TTBs toward the imports of non-PTA partners could be an important avenue by which PTAs lead to rising external tari¤s.

Finally, in addition to the literature we survey above, an active literature models the choice countries face between PTA formation and multilateral trade bargaining.<sup>107</sup> That literature is

> 105 Recent work by Mai and Stoyanov (forthcoming) provides initial evidence on this topic. They examine the e¤ect of CUSFTA on Canadian trade policy. Consistent with the tari¤ complementarity e¤ect, they ...nd that the CUSFTA led to declines in Canadian external tari¤s. But they also ...nd that Canadian external tari¤s declined more slowly in industries that generated more revenue for US exporters to Canada. This latter e¤ect suggests an attempt by Canada to limit preference erosion in industries of particular importance to its PTA partner (i.e., the US).

> 106 Bown, Karacaovali, and Tovar (2015) provide a more general survey on the relationships between PTAs and the use of temporary trade barriers such as antidumping and safeguards actions.

> 107 Other related research considers the endogenous network of stable PTAs when multilateral liberalization is not included as an option. See Goyal and Joshi (2006), Furusawa and Konishi (2007), Mrázová, Vines and Zissimos (2013) and Yi (1996). Another related strand of the literature considers endogenous formation of CUs using the core

66