It is very hard to not introduce bias into social sciences. This is also why psychology and software engineering studies are hard to trust. It is more pervasive in economics when there are rewards to be linked to a given worldview. (You can make a lot of side money as an expert witness if you have written a lot of papers either solidly pro or against anti-trust legislation)
The source you're linking is mentioned in the first 3 words of the article he linked. The MotherJones.com article is a direct response to the article you linked.
http://fivethirtyeight.com/features/economists-arent-as-nonp...
It is very hard to not introduce bias into social sciences. This is also why psychology and software engineering studies are hard to trust. It is more pervasive in economics when there are rewards to be linked to a given worldview. (You can make a lot of side money as an expert witness if you have written a lot of papers either solidly pro or against anti-trust legislation)