Why are there so many PhD positions and so few PostDoc positions?

Question

I don't have hard numbers (if you know a good source, please do share), but it is quite apparent that there are many more Ph.D. student positions advertised than PostDoc positions, at least in my field (engineering/CS, Europe). My feeling would be that many labs I know have a ratio of maybe 5-10 Ph.D. students to 1 PostDoc. Why is that the case?

Specifically, why do professors or funding organizations prefer hiring a Ph.D. student to hiring a PostDoc? I know that the latter are a little bit more expensive in terms of salary*, but they will generally also be much more productive. (I'm just comparing myself now at the end of my Ph.D. with myself at the beginning of it.) If I were a professor now, had enough money at hand and could choose freely how to spend it, I'd aim for a much lower ratio, maybe 2 Ph.D. students to 1 PostDoc. Why don't people do that? Are there funding constraints (if yes, for which reason)? Aren't there enough qualified applicants (I would doubt this)?

In the larger scheme of things, hiring more senior researchers and less Ph.D. students would also contribute to solving the much lamented (at least in Germany) problem that there is a lack of viable scientific career paths short of becoming a professor.

One final comment: in an earlier question of mine, someone replied that universities have a teaching obligation, thus effectively obliging them to hire Ph.D. students instead of PostDocs. However, in my country (Germany) and field, Ph.D. students are not really seen as students; they are essentially fully paid staff researchers. (They also do not visit any lecture courses.) I do not believe that supervising a Ph.D. student is seen as "fulfilling the university's teaching obligations", although I might be wrong about that.

EDIT: Since the question of ethics and "do people actually want to stay in academia" came up a few times, here are a few articles about the detrimental effects of job scarcity at the postdoc level:

• https://www.nature.com/articles/d41586-020-03235-y
• https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0250662
• https://www.nature.com/articles/d41586-021-01548-0
• https://www.sciencemag.org/careers/2008/08/science-careers-poll-results-realistic-readership

*This will likely vary by region. In another answer, someone claimed that PostDocs are twice as expensive as Ph.D. students. In my institution, however, the pay gap is much lower. Maybe around one fifth or fourth?

Answer 1

A few reasons from a European perspective:

• Graduated PhD students are a success metric: Institutions and departments might be evaluated based on their number of graduated PhD students. Likewise, when a professor goes up for a tenure evaluation or applies for a job somewhere else, they typically need to show a track record of successful supervision of PhD students. Successful post-doc supervision doesn't nearly count as much.
• Better success chances for post-docs: Successful supervision of PhD students is also a success metrics when post-docs apply for faculty jobs. A research group with a rather high postdoc-to-PhD ratio will give less opportunity for each post-doc to gain the necessary supervision experiences.
• Ethical reasons: A situation in which every PhD candidate can get a post-doc, but most post-docs have no chance for a professorship might be considered ethically dubious. A post-doc is a temporary position with the goal of strengthening the post-doc's research profile towards a successful application for faculty jobs. Since there are much more people interested in a PhD than there are available professor positions, there has to be a point where "the funnel is narrowed", as to avoid that too many hopeless candidates are stuck in the post-doc stage without any realistic job prospects.

Answer 2

Graduate students are being educated for a broad range of careers, mostly outside academia. There is, after all, a large demand for PhDs in many fields of industry and in government. Universities heed this outside demand by educating many of them.

Postdocs are being educated primarily for academic careers. There is a much smaller demand for them, and so universities hire fewer of them because they would otherwise end up with lots of well-trained postdocs who cannot find jobs for which they have been educated.

Answer 3

It's not an academic reason, it's because the labor supply is saturated pushing up qualifications people are chasing.

Answer 4

Hierarchy is everywhere, whether you like it or not:

1. there are more secondary school graduates than college graduates;
2. there are more Masters graduates than PhD graduates;
3. there are more Post-docs than Professors.

Answer 5

I would argue that the role of a postdoc is often different from that of a graduate student. I would categorize many such reasons as "managerial".

For instance:

• Post-docs may be expected to serve as a "back-up" or secondary advisor (sometimes in addition to employing full-time lab managers), particularly in large groups, where some post-docs may take on significant supervisory duties. Having too too many post-docs may result in conflict (too many cooks do spoil the broth).

• Some professors I have known have displayed distinct signs of "ageism", that is, a preference for younger applicants (purposely or not, this is often encoded in requirements that PhDs be granted only a few years before applying for the postdoc). The experience that seems so admirable can also lead to friction and confrontation under the wrong circumstances.

• Incoming students are malleable and often more patient than more experienced individuals. They are more willing to follow orders blindly and take on risky projects, in part because they are blank slates, also because they have more time to find and develop a successful project. Ph.D. projects are often supposed to be very high risk (at least historically, although this may be declining). Part of the strategy of discovery is to try something so wild nobody else has bothered to test it before.

• Postdocs may be highly specialized to either complement or enhance (through teaching) the skills of graduate students. They may be hired for a specific role (rather say than to chase whatever new lead strikes the advisors fancy). There is a natural flow of information here consistent with the educational role of the institution.