Inspired by a comment from SEBASTIAN RAMIREZ on a previous post here, I started thinking about people I've met who have not been cut out for PhD research in various ways. Before joining the TT, I would have though this was an easy thing to determine. However, experience has shown that there are many things that can make a person not cut out for PhD research, some of which are harder to see than others (for both the student and the mentor). The stereotype is that people not cut out for a PhD either lack technical/intellectual skills or are lazy, and therefore this can be determined right away. However, I find that this is not usually why people leave our PhD program without a PhD (though it does happen, usually in the first year).
Lack of desire: This is the most common reason people leave our PhD program. I've seen plenty of people who like the idea of research but not the reality--some don't like the uncertainty (is there even an answer? will I ever find it?), some don't like the repetitiveness required to ensure reliable data, some don't like the large problem solving component, some don't like the required background reading/lit review and just want to do experiments. Some people think they like doing science, but actually just like reading about discoveries. Some people did well in their science classes and apply to grad school because they don't know what else to do (and at least they can get paid). Some people apply to grad school due to family pressure. Some people apply to grad school because it provides a way out of their country. Some of the people who start a PhD program without a direct desire to do research discover they enjoy it. Some don't.
People can fool themselves for quite a while about what they actually want, so students can be pretty far along when it becomes clear that they don't really want a PhD and/or dislike research. Sometimes they attempt to tough it out anyway to try to not "waste" time sunk into a degree or because they don't
know what they want to do instead/family pressure/need the paycheck/don't want to leave the country. This doesn't always go well because it is difficult to keep working hard for something you don't really want when you also have no interest in the work.
Inability to mature as a researcher: Many of the students who leave our program with an MSc instead of a PhD have good lab and data analysis skills, but just cannot make the leap to doing PhD level work. This can be hard to spot until 2-3 years in, when students in the PhD program typically are driving their own projects. These students can design and complete experiments, but have difficulty deciding what experiments should be designed. The cliche way to put this is that these students can't see the forest for the trees, but this is not exactly it--they are not overwhelmed by details, or overly detail oriented, but they cannot see how their work fits into a big picture in a way that lets them run a project themselves. Sometimes PIs push these students through the PhD program, but that really isn't in their best interest, since future employers will expect PhD-holders to be capable of driving a project.
Need structure: This can be a huge issue. Students starting out get lots of help with planning and carrying out their research, plus they often start out doing some coursework. As students go further into a PhD program, students are expected to take on more and more of the planning and scheduling themselves. Classes (if any) are completed. Most of the days are wide open to get work done. Some people cannot be productive in an unstructured/unscheduled workday. Some people need deadlines--for these people, even the weekly deadline of group meeting or a progress meeting is not enough pressure to help them focus. Most PIs are not interested in having daily scheduled meetings with senior students (which can help people who need deadlines), but some will do frequent meetings. The lack of structured days needs to be solved by each person though. Sometimes, they just can't do it.