Supporting a graduate student costs approximately $50,000 per year, though students typically receive only about $30,000 in stipend (the remainder goes to benefits and tuition remission). The ideal situation is for students to have independent funding; a 3-year fellowship is equivalent to a $150,000 grant. Conversely, if a student joins the lab and is funded by an NIH grant award to my lab, they will be expected to concentrate their effort towards the aims of the grant. Especially in the first two years of graduate school, when students are dividing their time between lab and classes, it is challenging to devote $50,000 of an NIH grant to a student who is in the lab part-time, when the same funding could support a research technician who is in the lab 100% of the time. I can, and sometimes do, support graduate students from NIH grants but only those students who recognize the importance of productivity in the lab even when they have classes, seminars, and other responsibilities are likely going to thrive.
The flip side to demanding productivity is that I strongly feel this is in a student’s long-term interests. There are a glut of biology PhD students, with more students in graduate school than there are positions for PhD scientists in academia or industry. There are lots of great careers for students who obtain PhDs, but the most desirable ones will, on average, go to students who have the most impressive record of productivity.