I wrote the following on a closed Facebook group for Wellesley alums, and since I spent all that time writing it, thought it would be worth it to share with a larger audience.
See also: How to Fail Out of Grad School Without Really Trying (which I wrote while I was still in grad school).
I thought everyone in this group was tired of me talking about why grad school is a mistake ;) For me, grad school turned out not to be what it says on the tin. That is, I went (to two different Ph.D programs) thinking I was going to learn how to be a researcher. Instead, I found I was being evaluated on things that had nothing to do with ability to do research. My first program had a prelim that was basically filtering out anybody with any degree of impostor syndrome. My second program didn't want me there because I wouldn't tolerate sexual harassment (I wrote about that at length).
I also found that graduate programs -- compared to anyplace I've worked that wasn't a university -- are very unforgiving of, basically, any sign of humanity: chronic illness (regardless of whether it's coded as "physical" or "mental") that interferes with work or causes you to need extended time off. In my experience, if a company hires me, they value me and, all other things being equal, don't want to lose me. So they have a reason to accommodate me (not that companies are perfect about this, of course; the job I stayed at the longest, I left because my manager there made my disability the focus of my annual performance review). In a Ph.D program, though, you're disposable and interchangeable; you're paid very little, so in the same way you might throw away something cheap you bought at Target whereas if you invested in an expensive piece of furniture, you'd fix it, your department generally has no reason to keep you around when they see a way to replace you with somebody healthier.
Don't underestimate the effect of spending 5-6 years or more -- which are, for most other college-educated people, the very years they spend building their careers (often, the years when they can just focus on work and don't yet have a lot of family obligations) being severely underpaid. Of course, this is something that varies a lot by field, and if somebody is in (say) sociology I realize there aren't a surfeit of high-paying jobs outside academia. But for me (I'm in computer science), if I went back to grad school today, that would cut my pay to a sixth of what it is now. Even if you don't care about money (and I didn't think I did when I was starting grad school), it's easy to underestimate the psychological effects of being paid fairly vs. being grossly underpaid because your employer hopes you'll fall for the promise of jam tomorrow (that is, a tenure-track job, which in reality is all but unavailable anymore) and give them your labor for practically nothing now. It really changes the relationship between you and your employer when they're paying you enough that they value their investment in you and won't kick you out the door the first time you show you're human.
When you're considering opportunity costs, also make sure you understand the real costs of grad school. Obviously, don't go anywhere that won't pay your tuition in full and give you a stipend for living expenses. But also know what the cost of housing is where you're going to be going to school; whether you can afford to live near the university or whether you'll have to factor the costs of commuting in (both economic and psychological; having any length of commute to work is apparently a major cause of stress and unhappiness in people's lives); whether or not your program will cover your health insurance or will add insult to injury by claiming you're not an employee (at the university I left, I had to pay thousands of dollars a year out of pocket -- a huge percentage of my stipend -- because they deliberately hired research assistants at 0.45 FTE to avoid paying for benefits); and whether or not you'll have easy access to mental health resources (which some of the most stable and well-adjusted people I know needed after a couple years in grad school).
And then there's the question of what you're going to do afterward. I originally wanted to be a professor. After my second experience in grad school, I no longer did, because I didn't want to be like the professors in my department who defended a grad student who sexually harassed his colleague while blaming the victim. But even if that hadn't happened, I doubt I would ever have been able to find a tenure-track job; even though there are still some TT jobs in computer science (although they're disappearing like in every other field), they're reserved for only the people who scored highest in the privilege lottery and have a monomaniacal focus on work. One or the other (generally) doesn't cut it. In grad school, I never wanted to spend all my waking hours on research, which meant that if I'd graduated, I would have had at most 2 or 3 publications; when I read CVs for tenure-track faculty candidates who were coming to meet with grad students, they had as many as 20 publications straight out of grad school. I realized that I didn't like research enough to spend that much time on it, and in fact, I don't like *any* one thing enough to spend that much time on it. I'm passionate about more than one thing, and I'd even like to start a family sooner or later. I've talked to one too many people who had to choose between going hard for tenure and watching their children grow up, chose the former, and regretted it.
(edited to add:) If you already know you don't want to be a professor or work at a research lab, and you get a Ph.D, be prepared to have to remove it from your résumé to get a job; at least in computer science, a Ph.D actually lowers your expected salary compared to a master's. Be prepared to enter every job interview on your guard, explaining why you're not overqualified or why you won't be bored at a job that your interviewer is already assuming is "beneath you". (This is, at least, the experience of some of my friends with computer science Ph.Ds.) Some people describe a Ph.D as a "union card" to teach at a university -- if you already know that's not what you want to do, think especially hard about why you want to get one. At least in my field, everything else you can do along the way (teaching, learning, reading, writing) is work you can do outside a university -- often, as part of an industry job, while getting paid a lot more for it.
Now, just because grad school is essentially a huge scam that promises much in order to extract extremely cheap labor from grad students and make them feel like they're getting an education, that doesn't mean it isn't the right choice for some people. If you read all this and think "hmm, I really still want to go," then you should probably go. Some books you should read first, though:
Leaving the Ivory Tower, Barbara Lovitts: It's about why Ph.D students leave grad school (spoiler: the reason is usually structural in that a huge number of departments systematically identify a few favorites among each incoming cohort of Ph.D students and actively neglect the rest; but faculty take credit for their successful students while placing all the blame on the individuals who don't succeed).
Getting What You Came For, Robert Peters: very readable, and describes what you need to be doing to get through a Ph.D program. It won't help if your advisor is determined to defend sexual harassment, of course, but if you read this book and more-or-less do what it says I think that you'll avoid some of the major mistakes that are avoidable.
See also: How to Fail Out of Grad School Without Really Trying (which I wrote while I was still in grad school).
I thought everyone in this group was tired of me talking about why grad school is a mistake ;) For me, grad school turned out not to be what it says on the tin. That is, I went (to two different Ph.D programs) thinking I was going to learn how to be a researcher. Instead, I found I was being evaluated on things that had nothing to do with ability to do research. My first program had a prelim that was basically filtering out anybody with any degree of impostor syndrome. My second program didn't want me there because I wouldn't tolerate sexual harassment (I wrote about that at length).
I also found that graduate programs -- compared to anyplace I've worked that wasn't a university -- are very unforgiving of, basically, any sign of humanity: chronic illness (regardless of whether it's coded as "physical" or "mental") that interferes with work or causes you to need extended time off. In my experience, if a company hires me, they value me and, all other things being equal, don't want to lose me. So they have a reason to accommodate me (not that companies are perfect about this, of course; the job I stayed at the longest, I left because my manager there made my disability the focus of my annual performance review). In a Ph.D program, though, you're disposable and interchangeable; you're paid very little, so in the same way you might throw away something cheap you bought at Target whereas if you invested in an expensive piece of furniture, you'd fix it, your department generally has no reason to keep you around when they see a way to replace you with somebody healthier.
Don't underestimate the effect of spending 5-6 years or more -- which are, for most other college-educated people, the very years they spend building their careers (often, the years when they can just focus on work and don't yet have a lot of family obligations) being severely underpaid. Of course, this is something that varies a lot by field, and if somebody is in (say) sociology I realize there aren't a surfeit of high-paying jobs outside academia. But for me (I'm in computer science), if I went back to grad school today, that would cut my pay to a sixth of what it is now. Even if you don't care about money (and I didn't think I did when I was starting grad school), it's easy to underestimate the psychological effects of being paid fairly vs. being grossly underpaid because your employer hopes you'll fall for the promise of jam tomorrow (that is, a tenure-track job, which in reality is all but unavailable anymore) and give them your labor for practically nothing now. It really changes the relationship between you and your employer when they're paying you enough that they value their investment in you and won't kick you out the door the first time you show you're human.
When you're considering opportunity costs, also make sure you understand the real costs of grad school. Obviously, don't go anywhere that won't pay your tuition in full and give you a stipend for living expenses. But also know what the cost of housing is where you're going to be going to school; whether you can afford to live near the university or whether you'll have to factor the costs of commuting in (both economic and psychological; having any length of commute to work is apparently a major cause of stress and unhappiness in people's lives); whether or not your program will cover your health insurance or will add insult to injury by claiming you're not an employee (at the university I left, I had to pay thousands of dollars a year out of pocket -- a huge percentage of my stipend -- because they deliberately hired research assistants at 0.45 FTE to avoid paying for benefits); and whether or not you'll have easy access to mental health resources (which some of the most stable and well-adjusted people I know needed after a couple years in grad school).
And then there's the question of what you're going to do afterward. I originally wanted to be a professor. After my second experience in grad school, I no longer did, because I didn't want to be like the professors in my department who defended a grad student who sexually harassed his colleague while blaming the victim. But even if that hadn't happened, I doubt I would ever have been able to find a tenure-track job; even though there are still some TT jobs in computer science (although they're disappearing like in every other field), they're reserved for only the people who scored highest in the privilege lottery and have a monomaniacal focus on work. One or the other (generally) doesn't cut it. In grad school, I never wanted to spend all my waking hours on research, which meant that if I'd graduated, I would have had at most 2 or 3 publications; when I read CVs for tenure-track faculty candidates who were coming to meet with grad students, they had as many as 20 publications straight out of grad school. I realized that I didn't like research enough to spend that much time on it, and in fact, I don't like *any* one thing enough to spend that much time on it. I'm passionate about more than one thing, and I'd even like to start a family sooner or later. I've talked to one too many people who had to choose between going hard for tenure and watching their children grow up, chose the former, and regretted it.
(edited to add:) If you already know you don't want to be a professor or work at a research lab, and you get a Ph.D, be prepared to have to remove it from your résumé to get a job; at least in computer science, a Ph.D actually lowers your expected salary compared to a master's. Be prepared to enter every job interview on your guard, explaining why you're not overqualified or why you won't be bored at a job that your interviewer is already assuming is "beneath you". (This is, at least, the experience of some of my friends with computer science Ph.Ds.) Some people describe a Ph.D as a "union card" to teach at a university -- if you already know that's not what you want to do, think especially hard about why you want to get one. At least in my field, everything else you can do along the way (teaching, learning, reading, writing) is work you can do outside a university -- often, as part of an industry job, while getting paid a lot more for it.
Now, just because grad school is essentially a huge scam that promises much in order to extract extremely cheap labor from grad students and make them feel like they're getting an education, that doesn't mean it isn't the right choice for some people. If you read all this and think "hmm, I really still want to go," then you should probably go. Some books you should read first, though:
Leaving the Ivory Tower, Barbara Lovitts: It's about why Ph.D students leave grad school (spoiler: the reason is usually structural in that a huge number of departments systematically identify a few favorites among each incoming cohort of Ph.D students and actively neglect the rest; but faculty take credit for their successful students while placing all the blame on the individuals who don't succeed).
Getting What You Came For, Robert Peters: very readable, and describes what you need to be doing to get through a Ph.D program. It won't help if your advisor is determined to defend sexual harassment, of course, but if you read this book and more-or-less do what it says I think that you'll avoid some of the major mistakes that are avoidable.