It is not an assumption, and yes I can warrant the claim. I wonder if you can do the same for your own claims which precipitated my counterclaims: (1) that people always talk about how they hate gays and the gays' problematical existence; (2) that gays are always flaunting the existence of the gay community; (3) that the media would not report a falsehood; and (4) that common belief is a reliable indicator of veracity.
The basis for my own observations derives predominantly from the field of psychology, with additional research being contributed from fields such as biology, neurology, etc. The body of research is summarized well by Shermer, M. (2011). The believing brain. Scientific American, 305(1), 85-85:
We form our beliefs for a variety of subjective, emotional and psychological reasons in the context of environments created by family, friends, colleagues, culture and society at large. After forming our beliefs, we then defend, justify and rationalize them with a host of intellectual reasons, cogent arguments and rational explanations. Beliefs come first; explanations for beliefs follow. [...] Once we form beliefs and make commitments to them, we maintain and reinforce them through a number of powerful cognitive biases that distort our percepts to fit belief concepts. Among them are: ANCHORING BIAS. Relying too heavily on one reference anchor or piece of information when making decisions. AUTHORITY BIAS. Valuing the opinions of an authority, especially in the evaluation of something we know little about. BELIEF BIAS. Evaluating the strength of an argument based on the believability of its conclusion. CONFIRMATION BIAS. Seeking and finding confirming evidence in support of already existing beliefs and ignoring or reinterpreting disconfirming evidence. On top of all these biases, there is the in-group bias, in which we place more value on the beliefs of those whom we perceive to be fellow members of our group and less on the beliefs of those from different groups. [...] Belief-dependent realism is driven even deeper by a metabias called the bias blind spot, or the tendency to recognize the power of cognitive biases in other people but to be blind to their influence on our own beliefs. [...] This dependency on belief and its host of psychological biases is why, in science, we have built-in self-correcting machinery.
See also, for example: Merritt & Monin (2011), Wright & Baril (2011), Haidt et al (2001), Bechara et al (1997), Greene (2007), Hammerstein & Stevens (2012), Frijda et al (2000), etc.
The basic reality is that we are all of us (with some few exceptions owing to brain damage to the VMPC or other related areas) heavily influenced by subjective, emotional cognitive bias. Quite simply, most people not only fail to think critically but are biased against doing so. Even assuming they could overcome this, it seems rather an assumption that they would have the time and resources to systemically subject their beliefs to a rigorous process of critical thought. And, perhaps more importantly, most people would not need to - emotive belief satisfies the minimal evolutionary requirement, so it is good enough for the everyday person.
