Early research into group influence found that majorities have a strong effect on people. The classic study is Asch's line judgement task, where people were found to conform with the rest of a group (who were confederates) when they made a very obvious error in judging which of three comparison lines was the same length as a standard line. However, it was not until the 1970s that research began to focus on the way minorities can influence majorities. Previously, it was assumed that the role of the minority in persuading people to change their attitudes, beliefs, or judgements was minimal, if it existed at all. But there have been many real world cases of minority influence, such as the Civil Rights Movement in the USA and gay rights. In the UK, the Green Party was originally a minority, but now environmental issues have become a significant part of all Party policies.

It is clear that both minorities and majorities can influence people, so any theory of how groups influence people must account for this. The most influential theory of group influence is Moscovici's (1980) conversion theory. Moscovici's theory is based on the idea that if an individual is exposed to an argument, or some other form of persuasion, that is contradictory with a currently held attitude, then there will result a conflict. Conversion theory assumes that individuals are motivated to reduce this conflict, and how it is resolved depends on whether the source of conflict is a majority or a minority. If the source is a majority, Moscovici says that individuals engage in a social comparison process which involves them identifying what the majority's position is so that they can adjust their own position to conform. This is because being associated with the majority is desirable. For minority sources of influence, processing of the group's message occurs rather differently. Firstly, social comparison is unlikely because being associated with a minority is generally undesirable, since minorities are often viewed unfavourably. But due to the distinctiveness of the minority, Moscovici claimed that this elicited a validation process in individuals, which causes people to examine the minority's arguments in order to understand them and understand why these people do not believe what the majority does. With majority influence, people do not closely analyse the majority's arguments.

Attitude conversion can follow majority influence, but it is more compliance than true attitude change. As such, this compliance is public, and not private. The analysis of the minority's view, however is conducive to private attitude change. Mugny (1982) has suggested that people will not generally hold a minority view publicly as they do not want to be categorised as a minority member and be associated with any stigma that may bring.

Moscovici's approach is important as it has identified the types of cognitive processes that occur in group influenced attitude change. When a minority influences, the direction of an individual's attention is on the content of the message, rather than the relationship between themselves and the majority group. Leading on from this, the quality of the cognition is also different with minority influence leading to evaluation of the message and majority influence leading to evaluation of the social value of the group. Finally, there is a difference in the quantity of cognition, with minority influence leading to more cognition than for majority influence.

However, Moscovici's conversion theory is not unchallenged. Mackie's (1987) objective consensus approach is in many respects opposite to conversion theory. Mackie claims that it is the majority that elicits greater message processing as people assume that the majority view is probably valid based on the heuristic idea that "several pairs of eyes are better than one". Part of Mackie's idea is the "false consensus effect", which says that people believe they share similar views to the majority group and dissimilar views to the minority. If an individual is faced with a counter-attitudinal majority, it violates the expectation of consensus and thus there is much processing of the majority's message so as to understand the difference in opinion. According to Mackie, a counter-attitudinal minority is not surprising, and so people do not engage in a great deal of cognition over the minority message.

The objective consensus approach seems to suggest that minorities do not elicit much attitude change. Another theory that does account for the role of the minority in attitude change is Baker and Petty's (1994) source/position congruency model. Baker and Petty's model shares some similarities with Mackie's ideas, but extend them somewhat. They suggest that greater processing of a source message depends on whether it produces an unbalanced situation. So, greater processing occurs in the face of a counter-attitudinal majority, or a pro-attitudinal minority - i.e., imbalanced situations where expectations are violated.

The above theories can be classified as dual process theories, as they attribute majority and minority influences to different processes. However, more recent ideas have attempted to explain majority and minority influence in terms of a single process. Latané and Wolf's (1981) social impact theory views majority and minority influence in terms of the strength of the source (e.g., their power or expertise), the source's immediacy (their proximity in space and time), and the size of the source (i.e., the number of group members). Although this theory holds that majorities have more influence than minorities, it does allow minority influence if that influence scores highly on strength and immediacy. However, the problem with this model is that is does not explain how minorities (or majorities for that matter) produce attitude change, and it also makes the false prediction that consistent majorities will always be more influential than consistent minorities.

A better single process theory is Turner's (1991) self-categorisation theory. This theory holds that influence comes from individuals that are categorised as similar to oneself on a basis relevant to the topic of influence. Similar others are seen a providing validation of one's own views. However, if these similar others are in disagreement with an individual, then that individual will be influenced. As far as minority influence is concerned, self-categorisation theory suggests that to have influence, a minority must be classified as a subgroup of the individual's ingroup, and not as an outgroup. This is a different view to conversion theory, as it is minority similarity, rather than dissimilarity, that promotes attitude conversion.

There is no definitive evidence supporting one theory over the others. However, it appears that the idea that minorities promote more systematic processing of arguments is becoming favoured. Such ideas are supported by the message quality effect, which states that if systematic processing occurs, strong arguments should lead to greater influence than weak arguments. This effect has been found several times for minority arguments - such as Martin, Hewstone, and Martin (2003). It was found that minorities instigate systematic processing and thus attitude change was more robust to future counter-persuasion. The opposite was found for majority influence. Also, Martin and Hewstone (2003) found that the nature of the topic of persuasion plays an important role, which is also related to how much systematic processing goes on. It was found that if a majority advocated a position that was high in negative outcome, then this would encourage more systematic processing of the argument. This is because a majority advocating a position that costs them and is detrimental to them is surprising and cognition is required to understand why they hold such a view.

Martin and Hewstone suggest that neither majorities nor minorities inherently elicit greater systematic processing, but rather systematic processing depends on several factors which can be cognitive (e.g., distraction), motivational (e.g., personal involvement), related to the demands of a task, or the topic of persuasion (particularly topics that threaten self-interest). So long as a minority view is consistent (though not dogmatically inflexible), is low in negative consequences, and avoids any of the discussed factors that help systematic processing of a majority view, then minorities can be expected to influence, at least at a private level. Minorities do not influence that differently to how majorities do. Both rely on the systematic processing of their arguments to facilitate attitude change.

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