@Corinthian The difference is hard to describe in less than 5 pages. The PhD is a terminal, research-based degree. MA/MS doesn’t purport to produce independent researchers like the PhD does (though now you need years of post-doc too…).
Absolutely PhD programs can set off mental illness. There are a lot of situations and circumstances in grad school that draw out peoples’ insecurities. That grad school is also peoples’ first long-term full-time ‘job’ heightens the stress. Like how high schoolers feel out of place in college, grad students are gaining a lot more independence and responsibility vs their college days. The academic pool you’re in also narrows - 40 of the best students in the world in X subject are now all classmates - they all were probably the top student in their major at college, but now 39 of them feel like they’re failing grad school. It’s also not just about academic hierarchies or long hours for low pay - the mission of a PhD degree is to turn a fresh college grad into an independently thinking expert in a narrow field. That takes a lot of hard work and perseverance.
I’m a recent-ish STEM PhD graduate. 95% of my memories of grad school are really positive, but the PhD is long, tough and made me ask a lot of hard questions about myself for 5 years. Including like in the Atlantic article about ‘is my research useful to anyone other than me?’ That said, I don’t ever regret going to grad school. I had a really close group of friends and could nerd out 24h/day. It was an intellectual wonderland.
There’s a lot of uncertainty in research - you can spend years working on a problem and realize with one quick calculation that the method you were developing for the last 4 months is completely useless. You can work for 5 years on a problem and a lab from a different university publishes a more clever solution to what you were working on, and so you end up with a body of unpublishable work - by no fault of your own. You can get 4 years into your PhD and realize you don’t have the patience/hands/math aptitude/whatever for the exact area you’ve been working in (or even science in general), but it’ll take another 3-4 years to finish your PhD if you switch labs/sub-fields. Sometimes, research just doesn’t work - for any number of reasons. So does that mean your hypothesis is too hard to answer, or are you a bad scientist? Add on all the job search implications of research productivity/lack thereof during grad school.
It’s really easy to wrap up your self-worth into your success in research. And a lot of research ‘success’ is based on luck and timing. I had a wonderful advisor who encouraged really ambitious research but never forgot that the PhD is in the end an academic degree - to him, success was not just in getting the research done. Equally important was whether or not I was learning. That kind of advisor is unfortunately not as common as you might think, especially at the very top schools. Depending on your disposition, in order to avoid feelings of depression and anxiety, you’ll need a strong support network to keep reminding you that your value as a person is not related to your research progress.