How will constitutional showdown over gender laws play out?

Scotland’s parliament recently passed a bill making it easier for people in Scotland to “self-identify” and reassign their gender. Before the bill gains royal assent and becomes law it needs to be consistent with UK law as determined by Westminster. UK ministers have indicated they will block the legislation.

Why is a Scottish law about gender reassignment causing so much trouble?

The issue of trans rights is controversial in any setting, as demonstrated by the extraordinary way trans people and JK Rowling respectively have found themselves the target of so much vile abuse, especially on social media.