Does anyone know of an international postdoctoral association?
It's a great idea Scientist/Tommi but there's none yet that I'm aware of.  

A lot of issues are common to different countries, but there are many important differences too. A good reason to look up your national/sector/discipline association, and a possible roadblock to the establishment of an international group.

In Australia it's the EMCR Forum*. They recently asked about similar international groups that advocate for and represent early and mid-career researchers and got a few bites: https://twitter.com/EMCRForum/status/950530138491400197 


full disclosure: I am deputy chair of the EMCR Forum in 2018