加入多边贸易协定的新成员拥有类似的政治和经济体系。新成员和现有成员的规模和发展水平应当相似。作者的普遍结论是，pta可能会通过吸收与现有成员相似的成员来扩大规模。因此，PTAs不喜欢接纳那些具有不同国内特征的成员国(Mansfield et al .， 2013)。同样的理由是，这可能是希望加入的国家将被要求进行政治或经济改革的准入前程序的结果。
The new members who join the multilateral PTAs have similar political and economic systems. The size and the developmental level of the new and the existing members should be similar. General conclusion by the authors is that the PTAs may tend to expand by including the members who are similar to the existing ones. The PTAs thus does not like to stretch by the admitting of those members-states which have different domestic characteristics (Mansfield et al, 2013). The reasons for the same are that this may be as a result of the preadmission procedure under which the states who want to accede will be required to make political or economic changes.
Thus, it can be concluded that the research paper has been successful in meeting the requirements of the research. The discussion related to the expansion of Preferential Trade Agreement has been done after the proper research by doing the analysis of the literature and also by the empirical evidence. Thereby, the research paper answers the question of the research.
Thus, it can be concluded that, there is a trend of expansion in PTAs over the last 60 years. PTAs are important to the international political economy. Past studies have failed to focus on why some PTAs expand and others do not. In order to fill the gap that some of the PTAs expand while others do not. A key finding of the research is that the bilateral and Hub & spoke agreements rarely expand. Multilateral agreements do often enlarge. Another key finding is that the PTAs that add new members once are likely to do the expansion again in clusters in the global system. There is an evidence of economic and political uniformity in expanding PTAs.
In future, the research may be conducted on the explanation of expansion of international institutions. The result of the research is confined only to the PTAs or they lead to the expansion of wider range of institutions. Also, the research related to the bilateral PTAs is not limited to economic institutions only.