Is a joint science PhD/JD a bad idea if I end up pursuing academic science? Question. I'm going into my third year as a PhD student in oceanography, and due to my interest in law, I'm considering applying to the concurrent PhD/JD program at my university. I’m wondering whether this might damage my future prospects as an academic scientist. 

My background. I have long been interested in both law and science, but I'm not sure whether I would end up pursuing law, science, or both after graduation. My scientific research (computer models of the global carbon cycle) has implications for law and policy, but is completely independent from law and policy in itself. I think my background places me at the very top in being able to understand the science, but that is only one of my interests in law. My interest in science has not waned at all, and my scientific career is going pretty well. It’s my interest in law that has grown. 

The program. I've come to learn my university has a pretty flexible joint program (7 years instead of 4-5), and they’ve stressed that the course load can be made flexible to accommodate thesis research in my home department. At this point I believe I could maintain a level of scientific productivity even when studying law half- or three quarter-time. Financially, I think it's likely I could do the program for free, but worst case I might no longer be eligible for my $25K/year stipend. This would not be a dealbreaker, and I would find out before having to commit

My concern. My concern is that I don’t know exactly how I would want to use the two degrees. I'm interested in many different types of law. I would never be unhappy that I took the time to study law, if only to quench my intellectual interest. I would only regret the decision if I decided to pursue an academic career in science and this damaged by chances. Specifically, I'm worried that:


It could suggest a lack of commitment, i.e. a kind of 'Carl Sagan effect', where a professional interest outside your area of research is counted as a lack of seriousness. 
A JD would obviously reduce my scientific productivity for a few years while studying. While I believe I can maintain some productivity, the number of papers, citations, etc. would necessarily decline over that period.
Interesting question.  There are so many factors to consider, including time, cost, workload, etc.

It does seem though that having credentials in both disciplines can give you both a unique perspective and a means through which you can influence public policy. Forward-thinking academic departments may see that as a plus.  Of course, others could see it as a lack of commitment to pure science, but I'm speculating.

You should consider that the merging of science and law is not uncharted territory. A great example is Vanderbilt's law and neuroscience initiative, funded by the MacArthur Foundation. See 
Vandy Law &amp; Neuroscience for details.

However, Vandy does not have a 'law and neuroscience' department, and I doubt that the law school or school of psychology (housing neuroscience) would specifically look for a candidate with both a PhD and JD.  But, it would certainly fit. This may be true of other institutions as well.