The etiology of a perceptual belief can seemingly affect its epistemic status. There are cases in which perceptual beliefs seem to be unjustified because the perceptual experiences on which they are based are caused, in part, by wishful thinking, or irrational prior beliefs. It has been argued that this is problematic for many internalist views in the epistemology of perception, especially those which postulate immediate perceptual justification. Such views are unable to account for the impact of an experience’s etiology on its justificational status (see Markie (2005, 2006, 2013), McGrath (2013), Siegel (2012, 2013a, b), and Vahid (2014)). Our understanding of what we have been told can also be affected by, for example, wishful thinking or irrational background beliefs. I argue that testimonial beliefs based on such states of understanding can thus be rendered unjustified. This is problematic not only for internalist immediate justification views of testimony, but also for some externalist views, such as the form of proper functionalism endorsed by Burge (1993), and Graham (2010). The testimonial version of the argument from etiology, unlike the perceptual variant, does not rest on the controversial hypothesis that perception is cognitively penetrable. Furthermore, there is a stronger case for the claim that testimonial justification can be undermined by etiological effects since, I argue, testimonial beliefs can be based on the background mental states which affect our understanding of what is said, and our states of understanding are rationally assessable.