Relevance of a PhD with a quantitative psychology group for applied math I first posted this question in academia.se, there it was suggested that I could try at the field's specific se. If it is off-topic for here, I will gladly remove it.
My background is in mathematics, with a main interest in mathematical statistics / biostatistics, and I am considering applying for a PhD position with a quantitative psychology group. The research itself can apparently be made statistically challenging towards the candidate's interests, which sounds appealing. I am not sure if I want to continue in academia after the PhD, but it would preferably be in mathematical statistics or biostatistics.
Would a PhD research with a quantitative psychology group take away this option or make it more difficult, even if the reseach itself is approached from a mathematical statistics view?
 A: when you say quantitative psychology, what do you mean?  That is a tremendously varied field. It ranges from things like computational psychology to doing anovas. 
In my opinion, the best way to support your career in academia is to create a strong publication record in your desired field of interest. What I mean by this is that if your field of interest is meta-analysis, a college of sociology will be just as happy to have you as a school of nursing. 
Dual appointments most certainly exist in academia and are common for statisticians. 
A: To echo and further emphasize the statement of Yair Daon in the comments:
If your passion is developing/improving/analyzing methodology, it is important that your thesis will sufficiently reflect that. If your supervisor's expertise is in an application (e.g., psychology), s/he might not be interested in you putting your time into such methodology-heavy investigations. Also, s/he might not be able to adequately supervise you in those studies. If your PhD position is financed by a grant, be sure to check that the grant and its deliverables/work packages are focussed on methodology. Otherwise, you might end up doing simple analyses that don't challenge you at all.
If you want to have some application to your PhD work, then working in a non-mathematics/non-statistics department has an advantage because a mathematics/statistics department might be completely uninterested in practical applications. However, it needs to be crystal clear before starting your PhD that you are actually going to work on methodology and who is going to supervise you in this task.
Side remark: you state that the your interest lies in biostatistics, why going for a psychology PhD?
