Self-categorisation theory is Turner's modification of Tajfel's Social Identity Theory. Social Identity Theory was original created to explain relationships between large groups - such as social classes and categories.
Turner, in his reworking of Social Identity Theory, gives a primacy to the source of information. He states that in order for information to have influence, it has to be seen as valid. How we validate information is dependent on our relationship with the group that produces the information. Do we self-categorise ourselves as a member of the group providing that information? If the group identity is important for us, then we will be more accepting of the group's norms.
This is distinct from normative influence. Whereas normative influence just relates to opinion formation, self-categorisation theory relates to the validation of information. Normative influence can not in itself explain why we would be skeptical of overtly biased or physically or mentally impaired group members. If in the Asch experiment a number of the group was blind, we would be unwilling to self-categorise with that group.
Referent Informational Influence
Turner argued that we should abandon the normative/informational influence distinction in favour of what he called referent informational influence. This is informational influence that is dependent on the nature of the group that it came from.
Nevertheless, some psychologists are skeptical about this claim. Surely normative informational influence has a role to play in some situations?