These are very good points. But there might be some cases where the person needs to be told more bluntly, to save them years of fruitless work.

'who is she tell you? 'how would she know' -- someone very experienced; e.g., supervised many students to completion. Example: let's say a project requires advanced mathematics, and the student's level is only at the high school level.

@VitaminE, I doubt she's very experienced. If she was, she would have known better that no one really knows if someone "has what it takes" to do something. In your example she should say the person "needs advanced math", saying "you don't have what it takes" means that person is inadequate permanently.

@toby544, I don't think it's the authority of the supervisor to decide for the student how they spend their life. The student should decide if he/she want to work harder to make it happen. Being blunt is good, assuming you state facts. In this case it's nothing more than an opinionated personal insult.

This is wrong because it is the advisor's job to know and assess the student's abilities.

I dissagree... While in many situations one may not know if one "has what it takes", at least in my area (pure mathematics) there are cases where one knows without doubt that "one doesn't have what it takes".

@NickS, thanks for your opinion. I disagree with it. You only know they don't have now the abilities to complete their PhD. They may develop them in the future.

@AnonymousPhysicist, I disagree. My assessment of my PhD students are private to me. And they can change in time. The supervisor has made a clear mistake: she claims she holds the knowledge of the permanent, eternal inherent "abilities" of the students. But her authority is to assess the track-record of the student and whether it's sufficient to continue in the PhD, not to assess his/her inherent consistent traits and future abilities.

@Dilworth Again, this may be special to my area, but in my oppinion, a PhD in pure math requires some skills which one does not trully develop, one either has or does not have. This seems to apply only to pure math, but it does require such skills... And while in theory any student not possesing these skills should simply not get into the graduate school, the system is not perfect and from time to time such a student ends up in graduate school.

@NickS, yes, I know exactly what you meant, and I disagree with it. There is no "inherent skill" that a grad student accepted to a pure math program either "possesses or not", and moreover the idea that you as an advisor have a way to conclusively establish whether "they have it or not" is even more wrong. There is no evidence for these assumptions. Supervisors should stick to helping and providing factual advice and assessment on past achievements. Not to predict the future or perform cognitive profiling.

@Dilworth let me make a guess here: is it possible that you are not familiar with hard scientific fields such as math, physics and such? Assuming this to be true (and coming myself from physics) I would turn it around (with a bit of sarcasm ;) : who are you to tell whether someone in a field - presumably different from yours - can make a clear judgement on students or not? ;) I agree with Nick, math is a pretty clear case sometimes. Like olympic swimmers: sorry, many don't - and never will - have the prerequisites (say body, in math the brain) to be one. Never. Hard, but true.

@NickS Alex Bell hated math, Charley Darwin failed, Mikey Faraday was never actually taught higher education. They would have all most likely been told "you just don't have what it takes, quit now"

While I want to agree with this answer, I wonder who if not the research advisor should express concerns if a student is not on a trajectory to graduate (and if your answer is "nobody", then I disagree with you - I have seen way too many students waste way too many of their best years in grad school before eventually quitting, long after it should have been clear that they were riding a dead horse).

This answer is pure applesauce. If a student clearly does not have what it takes, it is an advisors duty to make this unequivocally clear and prevent years of wasted time and effort.

@Dilworth While I agree with your response to my comment, your response is not consistent with the question. Supposing the advisor says "you do not have what it takes," we cannot be sure if the advisor is talking about "inherent abilities" or "current abilities" or "abilities relative to other students I could be supervising" or if it's an excuse and actually money is the issue.

@AnonymousPhysicist, yes, I agree with you on this. The problem was the precise insulting term "you don't have what it takes" used, if indeed she did (as I questioned in my answer). This amounts to a cognitive profiling which is not in the remit of a supervisor. If she was saying the alternatives you presented it would be perfectly appropriate IMO. But NickS comment proved that these insulting terms are seen (mistakenly) as appropriate by many supervisors these days. So my answer went on to clarify this point.

@Mayou36, nice guess, but wrong. If you bothered to look in my profile you will have seen that I'm a professor in STEM, well acquainted with pure math.

@xLeitix, I gave a precise answer to your concerns in my answer.

@eps, thanks for your opinion. I disagree with it. As explained, it is not the remit of a supervisor to engage in cognitive profiling of students, and supervisors also don't have the ability to do so. What they can do is only asses whether the student is fit to their lab/project, and whether they would finish in time given their track-record until this point.

I'd give this +100 if I could. Take a look at any highly-competitive field and you will see a wide range of raw talent levels. Those will more raw talent don't have to work as hard, while those with little raw talent have to work their backsides off, but with a sufficient mix of raw ability and hard work and the right opportunities one can succeed. The idea that some people "just don't have it" is just plain wrong. They might need to work harder--sometimes much harder. But that can if they want it badly enough and have the right tools to do the work.

Certainly trying harder isn't enough; they have to try harder the right way, and they may not be able to make the current deadline, but that doesn't mean they can't do it if they want it badly enough, at least in general, unless they're so far behind that there's no way they can ever find a niche from which to compete. But in general I think the phrase "you don't have what it takes" gets thrown around far to freely, when the reality is actually either "you don't have the right attitude" and/or "I don't know how to mentor you".

@CGCampbell All your examples are examples of people with issues that easy to fix. I actually had students in the past that failed and I recommended for graduate school, failing or hating math or missing the pre-requisite is not the issue, and I would not tell such student that it does not have what it takes.... What I reffer to is the students completelly lacking any (so called) critical thinking.

@Dilworth I mostly agree with you, but I think you are overstating things a bit. "Supervisors should stick to helping and providing factual advice and assessment on past achievements. Not to predict the future or perform cognitive profiling." Surely it is OK and helpful for a supervisor to say their opinion, if they do it considerately and moderately. I agree with you about you don't have what it takes, and it is helpful that you have pointed out the problems with people saying this, though whether it is an "opinionated personal insult" probably depends on tone and context.

@toby544, yes, I agree that supervisors should assess the future of the candidate, but only with respect to the limited PhD timeline and their own project. They're not there to provide uninvited haphazard cognitive profiling. Of course, if they say things in a nice humorous or reserved tone this may be acceptable at times.

I've noticed in my travels that the idea that "inherent talent" is immutable, and is also the most important factor in predicting success, is quite dependent on culture. I have much preferred being among people who tend to allow the elements of hard work, and of luck, greater weight than do the people of my own culture. I have noticed that those of the cultures that I prefer tend to be happier in their work, and more creative, and even (this is top secret) more successful.

I believe that the OP's supervisor may be, ironically, somewhat less able than she believes herself to be, at least in that I think she has overestimated her ability to predict the future. I think it likely that she is unconsciously repeating, and perpetuating, something of a myth.