In 2004, Scott Baier and Jeffrey Bergstrand published that there were three economic determinants critical to the formation of PTAs. Countries are more likely to participate in PTAs if they have low transportation costs and larger economies. Third, countries with similar economic sizes are likely to benefit the most by forming PTAs. Economic determinants like
GDP, similarity of economic size, and distance between countries correctly predict over 80% of PTAs in effect as of 2020. The remaining PTAs can be attributed to political predictors. Countries under democratic rule are more likely to participate in PTAs than those under autocratic rule. Autocratic rulers are not elected, and thus do not have their power threatened by dissatisfied citizens. Democratic leaders are incentivized to keep their constituents satisfied, and PTAs can help lower the price of consumer goods. Advocating for PTAs also lets democratic leaders signal to voters that they are committed to policies that improve their welfare. Countries are also more likely to join PTAs if competitor countries have already done so. ==List of preferential trade areas==