See the first NYTimes column from the Standup economist.
Well, it's not a more rigorous proof of how trade can make the society worse-off (some may have already seen his "constructive proof"), it's his empirical "proof" of how economics can make the society worse-off. In a nutshell, he basically summarized some key findings from his recent JEBO paper:
1. Econ majors donate less than non-econ majors; (well documented, though)
2. Selfishness is not nurtured via taking econ classes among econ majors - taking econ classes did not have a significant negative effect on later giving by these students; (the "selfish gene" may have guide them to choose econ as their major in the first place)
3. Selfishness can be nurtured via taking econ classes among non-econ majors - taking econ classes did have a significant negative effect on later giving by these students. (negative externality)
Conclusion. For the society as a whole, taking economics classes makes it more selfish. So is economics a dismal science? Yes indeed, but moreover, it's kind of evil too.