Anxiety: A Very Short Introduction - Daniel Freeman, Jason Freeman 2012

What causes GAD?
Generalized anxiety disorder

Generalized anxiety disorder is a relatively new concept, appearing in the Diagnostic and Statistical Manual for the first time in 1980, and only really coming into its own as a diagnostic category in the 1990s. This partly explains why research on worry is a relatively recent development, with no single account dominant.

That said, several theories of worry have been influential, and we begin this section on the causes of GAD by discussing the four main ones.