Yes, motivated reasoning is certainly a danger for originalism and should be avoided. But that hardly represents a strong critique of originalism for at least two reasons. First, all decisionmakers are subject to motivated reasoning. Therefore, any method of decisionmaking that seeks to constrain judges from deciding cases as they wish will be subject to this danger. For example, if living constitutionalism requires judges to decide cases based on prevailing modern values, rather than the values of the individual judge, then motivated reasoning might lead the judge to believe his values are the prevailing ones. Similarly, if judicial decisionmaking is supposed to decide cases based on the law rather than who the parties are, but if the judge is more sympathetic to certain types of parties (employers over employees, labor unions over corporations, etc), the motivated reasoning might lead to mistaken decisions.
Second, there are methods for checking motivated reasoning. One of the best, I believe, is forcing the judges in the majority to articulate their reasons in an opinion and then allowing the judges in the minority to criticize that reasoning in a dissent. This can expose weak arguments to the public for all to see and works to check conclusions reached by motivated reasoning in the first place. Of course, this is not a perfect check, but no check is.
This last bit is sharp, and fits into some of my recent works:
Lot’s of people claim to be originalists, but whether they are actually practicing the approach, as opposing to posing or muddling it, is always an important question to ask.