Over the years, as I gained more experience as advisor, I realized that I could successfully advise a much wider range of students than I had initially thought. We faculty start out more-or-less thinking all graduate students are like us, motivated to the same extent and by the same things. Experience quickly disabuses us of this fallacious notion. People do a PhD for many reasons, and some whose reason is “I want to land lucrative corporate employment” do a bang-up job on their PhD, while some with nominally lofty academic aspirations cannot, in fact, do research successfully at all.
But I think experience has also made me too relaxed, because I started believing that, if I sufficiently personalized the mentoring strategy and project to the individual, I could bring almost anyone to a PhD.
Almost.
It turns out, I was mostly lucky, and my luck has run out.
The past 2-3 years have jolted me out of my complacence. People need to have high aptitude and be genuinely motivated and interested in the work. Not just pay lip service to their aspirations, but work hard and keep reading and thinking and struggling and pushing things a bit further every day. Initiative and self-reliance are critical. In my experience, the students who look like they’re limping out of the gate generally keep struggling regardless of the amount of feedback or the personalization of mentoring style and project choice. They generally do not have the capacity to do so independently enough, and the advisor and/or group mates have to carry them over the finish line. There are exceptions to this, of course, but the one exception I encountered cost me far too much time and money sunk into a majority who were not.
I need to be more resolute about putting a timely end to things that aren’t working out. I don’t want to carry another person over the finish line.
xykademiqz 
This really strikes home for me – with experience, getting too confident / complacent about taking anyone across, and eventually getting a rude awakening. Exactly as it happened for me. With the first (and hopefully last!) student who had to be carried over, I sunk in a huge amount of time and effort, and also peace-of-mind and equanimity.
Posting anonymously, lest said student sees this.
Oh snap, I was just looking forward to finally having gained some experience to tailor my supervision skills to each individual student and now it turns out there is another trap ahead.
Please inform me as to how long the “happy and experienced” phase lasts so I can mentally prepare.
I had one PhD student like this, it took them 7 years to finish (as opposed to the normal 3-4 years here in UK) and an enormous amount of energy from me. Here in UK, funding usually goes to students, not supervisors, so once someone is in, there is basically no way to get rid of them, unless they decide to stop. This one problem student didn’t want to stop, so we had to go through years of frustration and annoyance and sub-performance with no way out. If anything, this made me more cautious when I accept new PhD students and I firmly reject anyone with the same red flags as that problem student (which I didn’t see at that time as I was a fresh and inexperienced supervisor). Never again.
The lack of enthusiasm and motivation is my biggest complaint. The lack of aptitude? I’m used to it. But reading this dredged up a memory of a temporary job that I applied for before starting grad school. When I interviewed for it, the guy described in very clear terms all the miserable conditions I would have to put up with, and said he didn’t want to hire anyone who wasn’t enthusiastic and couldn’t put up with the difficulties. I told him I really wanted the job, and as the words left my lips, a little voice in my head said “Are you crazy? You don’t want this job!”
I was offered the job, I took it, was miserable for four months, and when I had the option to leave the project early, I took that. There’s a moral in here somewhere.
I thought, too, that enthusiasm and motivation were enough. They just aren’t. I realized that when I had a student who, despite hard work and enthusiasm, could never do the work. Changes in project, style of advising, none of it worked. The gaps in their knowledge were too numerous and too pervasive, and I think the gaps were there because the aptitude was never really there. They never formed logical connections between concepts into a solid framework into which new pieces of information could fit, thereby filling out and enhancing the overall structure. They presumably did not pick up things in undergrad and I witnessed them not picking up things in grad because they did not have the needed aptitude to pick up these things. After several years in grad school, a person has to be at the level where we are no longer plugging gaping holes in preparation, but they never got there. This person could never think and operate as a independent scientist. It didn’t help that their math skills were at best average, which doesn’t cut it in a theory group. They have other qualities, but I don’t think they should’ve been admitted to a PhD program, and they definitely should’ve never been a member of my group. I know it’s taboo to talk about aptitude, but not talking about it denies reality. Some people will never be able to do the work I need them to do, no matter how much time and energy (and funding) I sink into them. And it’s a disservice to those can do the work within a reasonable time and with reasonable effort to have precious resources siphoned by those who cannot.