Why do departments fund PhD students instead of postdocs?

Question

If I'm not mistaken, most PhD students (at least in the sciences, which is the field I'm most interested in) are funded. They're provided with a stipend, and all tuition fees are waived. In other words, it's effectively a low-paying job for them. My question is, why fund PhD students instead of postdocs?

I'm dismissing the altruistic reason ("we fund PhD students as a service to the community") immediately since it doesn't make sense - presumably if someone is spending money in this fashion, they'd fund need-based scholarships. The only other reason I can think of is that the department wants something out of the PhD students - presumably research output; can't think of anything else - that they think they're getting a good deal on.

Now according to Google, the typical PhD stipend is about $20-25k / year. Meanwhile, the typical postdoc is paid $47k / year. That means that a department can hire approximately one postdoc per two PhD students. To that we can add:

1. Postdocs have already been trained; PhD students are in training. Postdocs should hit the ground running while PhD students take time to get up to par.
2. PhD students especially in 5-year programs spend the first couple of years taking courses, i.e. not doing research work.
3. Postdocs can do other things like supervise Masters students that PhD students can't.
4. PhD students apparently have a high attrition rate, as high as 50%. Postdocs have been in the business longer, so presumably are also more aware of what they're going into.

It seems more sensible to me that departments should concentrate all funding on postdocs, and leave PhD students to pay for their own education. This is already the case for undergraduate studies. Why do departments continue to fund PhD students?

Answer 1

"I'm dismissing the altruistic reason"

Therein lies one of the biggest problems with your question. I work for a land-grant state university in the U.S. Our mission includes education. It's not an altruistic reason, it's literally why we exist.

The other flaw in your question is assuming that the money to hire these two groups is fungible - it isn't always. For example, there are grant programs where education of graduate students is a major component of what one is supposed to be doing - "we're going to hire a postdoc" is simply a non-alloweable use of funding.

There are some other nice features of graduate students to consider as well:

• They're cheaper. While a postdoc is probably more value for complex activities, there are sometimes things that require some expertise, but not a full-blown PhDs worth. Masters and early-stage PhD students are excellent people to conduct literature reviews, field collection, etc.
• You can identify promising researchers early. Lets say you see someone in undergraduate courses whose impressed you. If you only recruited postdocs, you're essentially saying "Look me up in 6 to 8 years, I'd love to work with you." If you recruit graduate students, you can usher them into their research career.
• You get them for longer. Postdocs are (by and large) transient positions, and much of their time may be taken up by looking (rightly) for another position. In many labs, especially those without long-term technicians, graduate students may actually have more institutional knowledge.

Answer 2

One of the premises of your question is simply incorrect in the US. Since most of the funding for postdocs and PhD students in the sciences comes from grants and the usage of that money is specified by the grant and approved by the funding agency, departments can't simply decide to spend the money differently.

Furthermore, in the US, postdocs can be considerably more expensive than PhD students.

For a Postdoc you've got to pay full time salary, plus fringe benefits (typically 30% on top of salary) plus overhead on the salary and fringe benefits. For example, a postdoc might cost $50K in salary per year, $15K per year in fringe benefits, plus another $35K per year (54%) in overhead for a total of $100K per year.

For graduate students the costs include a stipend, fringe benefits (typically much lower for graduate students than postdocs, e.g. 2% at my institution), tuition waiver (varies a lot between universities), and overhead (on stipend and fringe benefits only, since tuition waivers are excluded from overhead.) A student might cost $25K per year in stipend, $1K per year in fringe benefits, $20K per year in tuition, plus $15K in overhead (58%), for a total of $61K per year.

In these calculations, a postdoc was almost twice as expensive as a graduate student. Also notice that the postdoc costs three times as much in overhead as the graduate student. Many program managers hate to see funds going to overhead. For these and other reasons, there's a widespread preference in the funding agencies for funding graduate students rather than postdocs.

Answer 3

Adressing your points:

1. I find this only partially true. The spread inside each population in competence is larger than the distance between the populations.

2. (+4.) PhD students - if motivated and good - will stay until you give them the PhD. Others (not motivated, not good) will leave early. Postdocs are less easy to control and may be more prone to doing politics. Typically the motivated postdocs will change when it fits them best (e.g. after having a good publication) and the others may stay

3. That is purely theoretical. Wherever I looked, PhD students had other functions in the lab, too.

Additionally:

• people can be assessed and trained during their PhD - if they are good they may stay.

Answer 4

There are several reasons, here are some:

• The most important one, IMHO, is that the reputation of the department depends a lot on how many good PhD students they can produce. This will affect the ability to obtain funding, attract (masters/undergraduate) students, etc.
• There are some grants that are only enough to fund PhD students, or that can only be used to fund PhD students.
• Advisor to PhD student can be considered academic parent, advisor to postdoc is just another employer.

It seems more sensible to me that departments should concentrate all funding on postdocs, and leave PhD students to pay for their own education.

In Computer Science (and STEM in general), if you don't fund PhD students, not many people will do it. If one can have a PhD offer from Stanford, s(h)e can also get a job offer from big five (Amazon, Google,...) with a six-figure salary. And you want them to work for free in 5 - 6 years?

Answer 5

A few additional points that I have not yet seen mentioned.

First, many funding agencies have an explicit preference or even requirements that the funding be used to fund PhD students rather than postdocs. If your main funding sources don’t want to give money for postdocs, it’s hard to have them.

Second, tenure-track faculty members are usually expected to shepherd some number of PhD students to their degrees as part of the expectations for receiving tenure. You can’t do that if you only hire postdocs.

That said, postdocs are useful, and having an excellent postdoc does “pay for itself” several times over in terms of cost.

Answer 6

Another point that hasn't yet been mentioned is that it's not always correct that research output is what PhD students are being paid for. Especially at large American universities, PhD students are paid for teaching, often for leading discussion sections rather than lecturing courses as the main instructor. A postdoc is generally too senior to serve as a teaching assistant in this way, so there is a constant stream of money from undergraduate tuition to graduate student stipends.

Answer 7

In general, postdocs are expected to be able to write good scientific texts and have some reputation in their field. In particular, they are expected to be able to attract their own funding by writing research/grant proposals. This is not how it should be: postdocs are good at research and should concentrate on it, "converting coffee into theorems" (or, more generall, money into research)! But this is how it is, and is unlikely to change.