Postdoctoral training in biology and medicine is becoming common practice for PhD graduates. Over the thirty-year period between 1978 and 2008 the number of postdoctoral researchers (postdoc) has more than tripled from around 11,000 to more than 35,000. The number and demographic characteristics of these postdoctoral researchers and the length of time they spend in such positions is of great policy interest for funding agencies such as the National Institutes of Health (NIH).

The availability of funding for postdocs from multiple sources has made them attractive temporary research jobs, consequently attracting more non-American PhDs. In 1979, 73 percent of biomedical postdocs were US citizens, but this number dropped to 48 percent in 2008. This gap has been filled by international postdocs who help US academics by generating high quality research outcomes and serve as a source of high quality experts who might stay in the US. However, as international research opportunities change, it is reasonable to ask what future trends might be and how might that affect the availability of highly trained researchers working in the US.

In this study, we use the population level NSF survey of biomedical doctorates and develop a differential equation based-model of the supply and demand for US and international postdocs. We use the data for three main purposes: (1) as input to the model, (2) for model calibration and parameter estimation, and (3) to test and examine the fidelity of the model results to observed behavior in the NSF data set. Our model simulations are used to examine effects of four different policies: (1) capping the duration of funding for postdoc (2) drop in the demand for postdocs outside the US, (3) change in faculty hiring in the US, and (4) word of mouth in attracting international researchers.

To establish credibility we run simulations to replicate the data. Model projections show an increasing trend in the number of US and international postdocs, and a decline in the proportion of US to international postdocs.  A counterintuitive result from our model is that capping the duration of postdoc can increase the ratio of international postdocs. The reason is that, as our calibrated model estimates that international postdocs spend a shorter time in postdoc positions than their US counterparts.  Thus, capping the duration adversely affects more US researchers. Furthermore, as we cap duration of postdoc, more US postdocs leave and are replaced by new international postdocs, leading to even more international researchers than before.  

Our model also explores the effects of changes in the home countries of the international postdocs, which are considerable, as well as the effects of efforts to attract a more diverse group of researchers. The analysis suggests that the leverage point to affect diversity in the system is in the K-12 education stage, and policies implemented at the post graduate level have minimal effects on diversity, if any.