I'm an undergraduate student. I used to think that a PhD certifies a person as an expert in a specific topic of a specific field. I've now been told that while that's true, a PhD most importantly proves that you are capable of independent research in general.

A professor of fluid dynamics told me that if he wanted to shift his research to an unrelated topic like dog anatomy, rather than getting a second degree he would seek out successive projects that get progressively closer to his research interest (e.g. a project on modelling blood flow in a dog, etc.), until eventually he is working on his originally unrelated research interest.

Is the above approach generally valid? If it works, then what legitimate reason is there to get a second PhD?

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    In some countries, e.g. Italy, you are not even allowed to apply for a second PhD. – Massimo Ortolano Oct 11 '14 at 8:25
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    @MassimoOrtolano - Why not? What would be a reason for that? – Rook Oct 11 '14 at 8:29
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    @MassimoOrtolano, all my Italian friends in the PhD program at Oxford were also doing a PhD in Italy! They were using the Italian scholarship to finance their Oxford PhD. Perhaps what you mean is that you won't get a second scholarship for a second Italian dottorato. – PatrickT Oct 11 '14 at 8:47
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    @Idigas: I checked better, and I now see that my comment above is not entirely correct. Actually, in Italy you can apply for a second PhD, but you are not allowed, by law, to have a second State scholarship for it. Most of PhD scholarships in Italy are funded by State Institutions (privately funded PhDs are rare), and the State will not pay you a second scholarship. It appears, however, that if you have a job, and willing to take a second PhD without scholarship, you can (the cost of a PhD here is around $2000/yr for three years, which is not too much if you have a job). – Massimo Ortolano Oct 11 '14 at 8:54
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    @PatrickT: you're right, see my comment to Idigas above. – Massimo Ortolano Oct 11 '14 at 8:56

I would tend to agree with your professor: a Ph.D. primarily certifies you as being capably of creative contribution to research, and secondarily as an expert in a narrow sub-discipline. Combine that with the continually shifting landscape of the scientific frontier, are there is a great deal of flexibility in what a person with a Ph.D. may end up doing over time.

I have heard one of my close colleagues say that: "One way or another, in ten years time we can't be doing the same thing we are now. Either we will have succeeded and need to move forward, or we will have failed and need to try something else."

In such changes, there is usually a significant degree of continuity that allows one to "pivot" from one area for another. Like in your professor's example, there are a lot of ways in which dog anatomy and fluid dynamics are related, and it's natural that an expert in fluid dynamics might well be drawn to the parts of anatomy most relevant to their existing skill.

A nice real-world example of such a radical transition: Tom Knight made his name pioneering networks and computer architectures, then radically shifted into biology. There is a nice interview with him about his history and how he made the transition, which involved lots of re-education but not bothering with the formality of another Ph.D. He's also moved back and forth between industry and academia quite a bit.

That said, I could imagine some transition so extreme that it might require an entirely new apprenticeship, e.g., from astrophysics to medieval French history. But that sort of change would be a rather extreme an unusual example.

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It depends. Largely on is the first PhD recognized?

Consider someone doing a PhD degree in some field with rather low standards. Say, in politics, or medicine. You may end up being frowned upon if you are in contact with technology PhDs, who (apparently) have higher standards.

It will of course also vary from school to school. And people in medicine and politics will downvote this answer...

So in my personal opinion:

  • if your PhD was 3-5 years additionally to a masters degree, is from a highly regarded school, involved publishing several scientific papers, and is in a discipline such as CS, Physics, Math: don't bother doing another PhD
  • if your PhD was a "small" solution, maybe only 2 years after a bachelor, you didn't publish anything before (if at all), and your work was mostly summarizing and discussing what others wrote before: compare to standards in tech departments.

Your professor of fluid dynamics probably has a PhD of the first kind. Widely respected as capable of doing own research. But you know: not all PhD programs have such standards.

I've read of PhD "thesis" assignments that essentially meant transcribing some old medical work into modern language; which apparently many students outsourced this, because they couldn't even read the script anymore...

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  • "Consider someone doing a PhD degree in some field with rather low standards. Say, in politics, or medicine." Why do these fields have 'low standards'? – Accounting Mar 13 '19 at 1:26

Some doctorates are more specific than a PhD

Some tasks require specific qualifications for which a generic PhD may not suffice. For example a researcher working on neuroscience or robotic prosthesis may come from various fields of science, but (depending on your jurisdiction) can be prohibited to work on people independently before obtaining also a degree in medicine - no matter what skills they may already have, the specific degree is mandatory. Similarly, there are areas of sociology and politology where a reseacher would be well skilled in the relevant areas of law, but they may need to obtain a jurisprudence degree to be allowed to practice those skills.

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I suggest asking yourself this set of questions before engaging on a second PhD:

  • Do I find delight in long hours of profoundly involving, extended, and solitary study?

  • Have I achieved my greatest satisfaction in researching and writing long research papers in the previous PhD? Would I enjoy writing more?

  • Do I have a compact intellectual drive and curiosity that is becoming more concentrated in the next field of research or several related fields? (This momentum needs to be distinguished clearly and honestly from a drive to have a Ph.D. in order to obtain something else, whether an attractive job, a certain status, a sense of accomplishment, and so on.)

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    This doesn't answer whether second PhDs are necessary. – curiousdannii Oct 11 '14 at 10:15
  • You need to add a bullet, "are you willing to work for very low pay for another 5-6 years?!" Also, I concur, you didn't really answer the OP's question... – daaxix Oct 11 '14 at 15:43

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