There has been a really interesting discussion of the number of PhDs, the number of TT positions, and exploitation at potnia theron's blog. In thinking about the issues, I looked back at my thoughts on the subject from ages ago. I find that I agree with my younger self still: that making opportunity available to as many qualified applicants that can be supported with current resources is more important to me than the difficulty in finding a TT job. Let people roll the dice on a TT position if they want to. In my field, there are loads of non-academic opportunities, grad students don't make all that much less than a newly minted BA or BSc, and the grad stipend is livable if not luxurious, so the main cost of doing a PhD is opportunity cost.
I know that this is highly field specific, so there may very well be fields where the calculation is different based on the availability of employment, but I strongly feel that if we artificially limit the number of PhD seats, it will be the underrepresented and/or marginalized folks that will lose opportunity. I find that in my field few incoming students plan on an academic career, so I don't consider myself to be training future professors, just future scientists. I certainly do make sure my students know that many highly desirable (TT either teaching or research, corporate research science, National Lab staff scientist) are very difficult to get without a bit of luck, and that if they want those things, their job is to get qualified and have a plan B just in case. In any case, a PhD is only worth it if the experience is valuable to someone in its own right. Getting a PhD for the gold star is a waste of time.
I believe there are fields for which these things seem to be more precarious. I believe that there are fields for which many (most?) PhDs are underemployed, forced to leave the field, or otherwise unable to use their degrees in a way they would like, even in STEM fields. I've heard that some biomed/life sciences fields are particularly afflicted as a side effect of the rise and fall in NIH funding. I am not sure how I would feel if I were in one of those fields--my students have (thus far!) all found positions they are happy with that require their degrees. At the same time, I also wonder how generalizable this doom and gloom about overproduction really is.
It's kind of like the extended media coverage and gnashing of teeth about the stress level of high school students who are applying to Ivy League-type schools, when the vast majority of higher ed students attend local schools that are not so competitive, making these articles mostly irrelevant. I do see loads of comments, blog posts, and articles about overproduction of PhDs in my field, which does not match my experience with my students or in my department. Do we see the unhappiness of PhDs unable to find desirable positions coming from "pedigree" schools, while the silent majority are happy enough with their outcomes (note that ProdigalU is not a pedigree school), or is the doom and gloom appropriate across the education landscape? How can we know?
Stats from the professional societies I belong to suggest that PhD unemployment is very low. That said, many of the articles complaining about overproduction in my field say that many of these young employed PhDs are in temporary or underemployed situations. I know that my department only really looks at the first post-degree position, so it is hard to say what happens to those who take postdocs or internships. I do know what happens to my students and to others I've worked with at ProdigalU, but I am one PI and not statistically significant. The NSF data I played around with in 2010 is way out of date. People complain about the Survey of Earned Doctorates (SED), 2015 data published in 2017, but it is pretty much all we have for newly minted PhDs. I can say that I received a doctorate in the US, and I never saw the survey, so at least some of the questions about the reach of the survey are definitely valid. There is also the Survey of Doctorate Recipients (SDR), which follows some people from doctorate to age 76. The SDR is due for an update in May 2017, but the last posted data is from 2013 (published in 2014).
The SED there suggests that a high percentage of those with definite post-grad plans in physical or life sciences at the time of survey plan on a postdoc. Something like 40% of new PhDs are visa-holders, which likely influences their post-grad plans and choices. Postdocs are normal in my field for those who want a research-heavy position, even outside academia, so this is not too surprising. Looking at the trends over time, the percentage of new PhD holders in physical science with definite employment commitments is within the historical norm (65-70%) since 1994. The trend in life sciences is a decrease over time, from a plateau at ~70% to ~57%. There was a sharp 5% drop in 2007, which held steady until 2010, when a slow decline began that continues to 2014. I didn't have a position set when I finished my degree, but I wasn't looking all that hard and found one soon after, so I don't know how strong a statement this is, but it does suggest that things are changing in the life sciences for new PhDs. The percentage of those with definite commitments who are doing a postdoc is more or less unchanged for both physical science (~50%) and life science (~70%) over the period 1994-2015, suggesting that there are fewer life sciences postdocs available, leading to a decrease in those with firm plans at graduation. But is this the result of taking longer to find something, or a sign that new life sciences PhDs are unemployed?
We can check of the SDR for a better idea, at least until 2013. The SDR suggests that as of 2013, unemployment was ~2% across a wide range of fields, and about 3% of surveyed science PhDs have involuntarily left the field (average, highest was 7.4% for physics). I don't have time for a detailed review of the data, but a brief look at the main tables does not suggest a calamity of unemployment for PhDs across all fields, and especially in STEM fields. The percentage of employed PhD holders 5 years or fewer post-degree has held steady from 2010 to 2013, so the trend in postdoc commitments in life sciences from the SED does not show up in the SDR. It remains to be seen if there was a huge change in the upcoming data release.
Anecdotally, it does seem harder for people to find positions, which is backed somewhat by the data in the SED for life sciences, but PhD-holders do seem to be finding them. This is consistent with my experience at ProdigalU, where students are taking a bit longer to find something, but are still finding good jobs in our field. There is a general rise in employment uncertainty in the US right now, with contract and part time labor gaining on full time employment as the new norm, and the adjunctification of some sectors of higher ed is certainly a symptom of that. Coupled with the normalization of full or partial soft money TT positions in medical schools (and really, WTF! I don't understand how a position can be TT soft money!) and the overall reduction in support for basic research, many people are in a precarious situation. But this doesn't appear to reflect the majority of PhD-holders, and I don't think makes a strong case for a reduction in the number of PhDs.