1) You have an irrational LOVE for research
A PhD is and always will be a devotion of some of your best years to research. So you better love it. I mean really love it. There will be several late nights where you are breaking your head over what to do next, or trying to get a piece of equipment to work. I have seen only one thing truly motivate grad students out of those slumps, their desire to discover.
2) You enjoy challenging assumptions
The most successful PhDs I know have always worked on projects that are fundamentally challenging an assumption in their field. Essentially, these people have such a strong desire for the truth, that unless it breaks a thermodynamic law, they believe it is possible! Needless to say, this is how great discoveries happen, and how we have technologies that can do insane things, like controlling a living mouse with just light (if you don’t know about optogenetics yet – google it!).
3) You know exactly why you want a PhD
I know this sounds circular, so I’ll elaborate. I have seen too many of my peers join a PhD because it was the thing that the smart kids do after college. That is the exact wrong reason to do a PhD. You should do a PhD ONLY if you know exactly what you want to accomplish at the end of the PhD. Remember, the PhD is a path, not an end. So if you want to be a professor or a lab head at a national lab for example, those are the obvious reasons to do a PhD. But please, do not sacrifice 5 years to do a PhD just because you are the smartest kid in your class.
4) You have a desire to invent
This is distinct from the first point, where I say it is import to love research. There I am referring to the process. Here I am pointing at the result – the invention. A PhD is one of the best ways to have intellectual freedom to invent things you are passionate about. I have been fortunate to have had this opportunity several times and trust me, it’s an awesome feeling. If you are lucky, you invent something so critical that the marketplace licenses it from you and your impact translates from lab to the ‘real world.’ While this does not always happen, having the desire to produce is instrumental in having a successful PhD.
5) You enjoy the learning-teaching process.
Being a graduate student means you are constantly either teaching someone or learning something. Unlike other jobs, where you acquire a skill and produce a lot based on that skill. Research is quite different in that you have to constantly stay updated, read papers, learn new techniques or ideas, and so on. So even if you don’t want to be a professor after the PhD, you have to enjoy the process.
While the above points may sound general or obvious, it is vital to be honest with yourself about them before making the decision. This is especially true if you are thinking about doing a PhD after having worked for a few years (this is increasingly common these days). For those of you making this decision right now, I wish you luck and success!