He shows that the pre-PTA volume of trade is not a useful criterion for selecting a partner. The pre-PTA volume is equal to zero if the partner is an importer of the good sold to the home country and it is indeterminate if the partner is an exporter of that good. First, a large partner is more likely to satisfy the home country's import demand at the world price. Second, the home country is likely to gain more on its exports to a large partner country, because that partner is likely to continue importing from the world market after formation of the trading bloc. And since the partner charges a tariff on imports from the world market, the home country is more likely to improve its terms of trade by selling to the partner at the higher tariff-inclusive price if the partner is large.
Losses are similar but less likely, while gains are both more likely and the same or larger.
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