The bystander effect is usually explained as resulting from diffusion of responsibility and pluralistic ignorance.

Judge as of 2008. True: standard social-psychology textbooks and reviews of that period name diffusion of responsibility and pluralistic ignorance as the main usual explanations of the bystander effect. Mostly true: these two are among the main explanations, with a third (e.g. audience inhibition/evaluation apprehension) often listed alongside. Mixed: only one of the two is commonly cited as a main explanation. Mostly false: these are minor or seldom-cited explanations. False: the literature attributes the effect to entirely different mechanisms.

Set before the research began.

1. E1 GROUP INHIBITION OF BYSTANDER INTERVENTION IN EMERGENCIES Journal of Personality and Social Psychology (Latané & Darley; scanned copy on archive.org) · 1968

   “"Diffusion of responsibility" seems the mostlikely explanation for this result.”

2. E2 CHAPTER 3 Bystander Effects - Springer Springer (book chapter by Ø. Kvalnes)

   “The two main reasons for the bystander effect are diffusion of responsibility and pluralistic ignorance.”

3. E3 The Bystander-Effect: A Meta-Analytic Review on Bystander Intervention in Dangerous and Non-Dangerous Emergencies. ERIC (abstract of Fischer et al., Psychological Bulletin) · July 2011

   “This pattern of findings is consistent with the arousal-cost-reward model, which proposes that dangerous emergencies are recognized faster and more clearly as real emergencies, thereby inducing higher levels of arousal and hence more helping.”

E1, the original 1968 Latané & Darley study, names diffusion of responsibility as the most likely explanation for the bystander effect. That puts one of the two mechanisms at the base of the literature. E2, an independent but undated book chapter, says the two main reasons for the effect are diffusion of responsibility and pluralistic ignorance, which matches the claim. Because E2 is undated, it does not show that textbooks of the 2008 period used this framing. E3, a meta-analysis from 2011, points to other models such as arousal-cost-reward, so the two mechanisms appear to be the main explanations but probably not the only ones.