Even the best candidates have weaknesses and areas for development. When writing a (generally positive) letter of recommendation, is it appropriate/helpful to mention these, or will this count against the applicant?
Does the answer change depending on the candidate's career stage (i.e. whether the letter supports an application for a PhD, Postdoc or Faculty position) or by geographic region?
Even the best candidates have weaknesses and areas for development. When writing a (generally positive) letter of recommendation, is it appropriate/helpful to mention these, or will this count against the applicant?
Does the answer change depending on the candidate's career stage (i.e. whether the letter supports an application for a PhD, Postdoc or Faculty position) or by geographic region?
Rather than listing weakness, describe how the person would grow in the position. For example, let's say you are writing a letter of recommendation for your recent graduate student, J. Doe. Rather than saying:
Dr. Doe lacks experience running large research programs and programming in Shiny new language.
Write something like:
Dr. Doe's experiences with my research group showed their ability to run experiments and projects. Your project would be a great opportunity for them to expand their skills for managing larger projects. Furthermore, Dr. Doe did a great job analyzing data with old rusty program and I am confident in their ability to apply similar methods in Shiny new language as part of your program.
Or, something along these lines that are true.
Also, this answer is US specific. A US letter would almost never include a negative statement for a positive letter. I have heard that other countries are different.
If you don't feel you can write a good letter say no. If you feel you need to mention a weakness say no.
Your letter should be positive. If you cannot whole heartedly recommend someone because of weakness in an area, then don't recommend them. I very rarely see any negatives in recommendation letters.
When I do, I wonder if the student/applicant knows what one of their references is saying this about them, and could find a better letter writer if they knew. It's as likely to make the reader wonder about you than the candidate.
There is also a difference in a weakness and room to grow. You can phrase room to grow positively. "This person is ready to lead a lab/write more grants/etc." Especially if this is the next logical step in their career, it's a positive they're ready to step up to the plate so to speak.
The is primarily an American based answer.
Your recommendation letter should, above all, provide a truthful picture of the candidate. But you of course also know that other candidates will receive recommendation letters written by people who might shy away from mentioning weaknesses, so you don't want to exaggerate. The difficulty is not whether to include weaknesses or not, but to find the balance.
This means: if you have to write a recommendation letter for a good student, who at some point made a mistake, or have some minor disadvantage, then don't mention it. On the other hand, if this student never kept a deadline and always had to have things repeated 4 times, it would be silly to write a stellar letter. Also: if you give that student top recommendations, what are you going to write for students who are truly great?
A good exercise (also backed by science, see eg Kahneman: "Noise") is to perform a mental ranking of this student vs other students, and make your letter reflect that raking. Say you rank this student in top 20%, you could write that, give your reasons, and also state what it would take for this student to be top 10%. Then it also does not come across as you "slamming" a particular student.
When I am asked to write a letter for a student I cannot give good recommendation, I usually tell the student this, and recommend them to find someone else who can write them a better one. They usually appreciate the honesty.
I have seen several universities that mention you should not talk about your weaknesses in a statement of purpose. I think the same rule applies to recommendation letters. If the weakness you want to write about is critical and you don't feel good recommending someone because of it, then don't write a recommendation letter for that person. Otherwise, if it's not that important and you still want to recommend the person, don't mention the weakness and instead just don't give credit to that person for that weakness. For example, if someone is really talented and hard-working but can't work very well in a group, don't mention their working ability in a group and only talk about their talent, etc.
Everyone has a weakness, and since no one else is writing about others' weaknesses, you writing about that weakness makes the person look bad. In fact, the name of the letter is the recommendation letter; I suppose that means why you recommend this person to us, not why you don't recommend them, so talk about all the good things that make you recommend them and leave the weaknesses.
TLDR: Always ask about geographic-specific format and content expectations in the region you are about to send your recommendation letter to. Careers can depend on it.
What I have heard is that the expectations around reference letters vary heavily by geographic region and that the differing cultures and the unintentional "clashes" between the cultures can significantly impact academic careers. The example that I heard (from a reliable source) was the difference between letters in the US and the UK (in STEM). Apparently, it is traditionally expected in the UK that letters include comments about a candidate's weaknesses. Traditionally in the US, however, anything other than a very positive letter is viewed as a red flag. Any comment about a weakness would destroy the chances of obtaining a position and could be perceived by the candidate as back-stabbing them (if they found out about the negative comment).
Thus, there were apparently significant problems at one point about obtaining recommendation letters from professors in the UK for US positions and vice versa. Fortunately, however, there has now been enough cultural exchange happening over the past many years that professors in the UK and the US better understand the cultural expectations and practises in the two geographic regions, such that fewer problems occur (but it is by no means perfect).
Given such a situation between two countries that have a significant cultural connection already, it is important to always ask about the geographic-specific expectations regarding reference letters if you are not absolutely sure. I commend you for asking about what to do on this site and further recommend that you reach out to people who know the geographic-specific expectations before you send any letters.
A recommendation letter should generally be positive and reveal the student's best qualities and lay out where the student can further develop and gain from the new challenge they are applying for. However, you should also always be truthful as you will not help your student if they obatin a position that is above their abilities because of a dishonest application. If you feel you really cannot write a very positive letter for someone, it might be better to decline the letter rather than hurting the student's chances to make the next step in their career.
