preferences using a gravity equation and a country-year indicator for whether preferences are in effect.<sup>7</sup> Our paper follows a more rigorous identification strategy to obtain a causal impact, combining a triple-differences specification, highly disaggregated country-product-year trade data, and treatment indicators identifying countries and products benefitting from GSP LDC and AGOA in each year.<sup>8</sup> Accounting for product heterogeneity in eligibility and hence for the extent of tariff cuts and preference margins is crucial to attribute positive trade outcomes to the preferences.

Using country-product-year data from 1998 to 2006, Frazer and Van Biesebroeck (2010) show that AGOA significantly increased African countries’ non-oil exports to the US and the impact grew over time from 2001 until 2006. We make four key innovations in our study relative to theirs. First, and most important, by considering the period until 2017 we examine the durability of the impact of trade preferences and test whether benefits survive the erosion of preferences due to the end of the MFA.<sup>9</sup> Second, by considering a period starting in 1992 we estimate separately the impact of GSP LDC from that of AGOA, whereas they attribute to AGOA an impact on nonapparel products for LDCs from 2001 onwards which was in fact an impact of GSP LDC from 1998 onwards. Third, and also crucial, our specification controls for treated-group specific time trends to account for the possibility that pre-treatment trends differ for treated and untreated product-country cells, which helps to ensure that causal impacts of AGOA and GSP LDC policy

> 7 The three studies use state-of-the-art gravity equation estimation which accounts for multilateral resistance terms, time-invariant and time-variant unobserved bilateral heterogeneity, and zero trade flows. Kassa and Coulibaly (2018) apply quasi-experimental synthetic control methods to bilateral gravity estimation of the impact of AGOA.

> 8 Our study’s nuanced view of the impact of trade preferences complements some gravity equation studies’ findings. Herz and Wagner (2011) obtain a negative long-run impact on exports of GSP beneficiary countries that they argue may be due to distortions induced by the GSP on the countries’ economic structure related to administrative costs to comply with GSP ROO. But their “long run” involves estimating their gravity equation using only data every 5 or 10 years (instead of yearly data as in their “short run”), they do not consider a post-preference scenario as we do. They conclude GSP-type trade preferences are not an adequate tool to promote economic development. Ornelas and Ritel (2020) show an ambiguous impact of nonreciprocal trade preference programs on average and a strong positive impact on exports of LDCs that are World Trade Organization (WTO) members. The rationale is that for LDCs trade preferences complement the economic reforms required by WTO membership.

> 9 A recent study by Hakobyan (2020) using country-product-year data and a triple difference-in-differences specification shows that developing countries’ exports to the US decline in 2011-2012 following a temporary expiration of US GSP in 2011 (although duties collected during the expiration period were ultimately refunded).

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