Attitude polarization, also known as belief polarization, is a phenomenon in which a disagreement becomes more extreme as the different parties consider evidence on the issue. It is one of the effects of confirmation bias: the tendency of people to search for and interpret evidence selectively, to reinforce their current beliefs or attitudes. When people encounter ambiguous evidence, this bias can potentially result in each of them interpreting it as in support of their existing attitudes, widening rather than narrowing the disagreement between them.
The effect is observed with issues that activate emotions, such as political "hot button" issues. For most issues, new evidence does not produce a polarization effect. For those issues where polarization is found, mere thinking about the issue, without contemplating new evidence, produces the effect. Social comparison processes have also been invoked as an explanation for the effect, which is increased by settings in which people repeat and validate each other's statements. This apparent tendency is of interest not only to psychologists, but also to sociologists and philosophers.
Psychological research 
Since the late 1960s, psychologists have carried out a number of studies on various aspects of attitude polarization.
The effects of prior theories on subsequently considered evidence 
In 1979, Charles Lord, Lee Ross and Mark Lepper carried out an important piece of research on attitude polarization. The researchers selected two groups of people; one group was strongly in favor of capital punishment, the other group was strongly opposed to capital punishment. The researchers began by measuring the strength with which people held their particular position on the death penalty. Later, both the pro- and anti-capital punishment people were put into small groups and shown one of two cards, each of which had a statement about the results of a research project written on it. For example:
Kroner and Phillips (1977) compared murder rates for the year before and the year after adoption of capital punishment in 14 states. In 11 of the 14 states, murder rates were lower after adoption of the death penalty. This research supports the deterrent effect of the death penalty.
Palmer and Crandall (1977) compared murder rates in 10 pairs of neighboring states with different capital punishment laws. In 8 of the 10 pairs, murder rates were higher in the state with capital punishment. This research opposes the deterrent effect of the death penalty.
The researchers again asked people about the strength of their beliefs about the deterrence effect of the death penalty, and, this time, also asked them about the effect that the research had had on their attitudes.
In the next stage of the research, the participants were given more information about the study described on the card they received, including details of the research, critiques of the research, and the researchers' responses to those critiques. The participants' degree of commitment to their original positions were re-measured, and the participants were asked about the quality of the research and the effect the research information had on their beliefs. Finally, the trial was rerun on all the participants using a card that supported the opposite position to that they had initially seen.
The researchers found that people tended to hold that research that agreed with their original views had been better conducted and was more convincing than research that conflicted with their original views. Whichever position they held initially, people tended to hold that position more strongly after reading about research that supported their position. Lord et al. point out that it is reasonable for people to be less critical of research that supports their current position, but it seems less rational for people to significantly increase the strength of their attitudes when they read supporting evidence. When people had read both the research that supported their current views and the research that was conflicted with their views, they tended to hold their original attitudes more strongly than before they received that information. These results should be understood in the context of several problems in the implementation of the study, including the fact the researchers changed the scaling of the outcome of the variable, so measuring attitude change was impossible, and measured polarization using a subjective assessment of attitude change not a direct measure of how much change had occurred.
Role of group membership 
Social psychologists have carried out research on the effect of seeing oneself as part of a group on one's attitude towards oneself, the group and positions supported or rejected by that group. To briefly summarize, the research suggests that people are likely to accept the position that they believe their group holds, even when they have only just been put into the group and have yet to meet any of the other group members.
See also 
- Fine 2006a
- Lord, et al. 1979
- Taber, Charles S.; Milton Lodge (July 2006). "Motivated Skepticism in the Evaluation of Political Beliefs". American Journal of Political Science (Midwest Political Science Association) 50 (3): 755–769. doi:10.1111/j.1540-5907.2006.00214.x. ISSN 0092-5853.
- Kuhn, Deanna; Joseph Lao (March 1996). "Effects of Evidence on Attitudes: Is Polarization the Norm?". Psychological Science (American Psychological Society) 7 (2): 115–120. doi:10.1111/j.1467-9280.1996.tb00340.x.
- Brauer, M.; C.M. Judd, M.D. Gliner (1995). "The effects of repeated expressions on attitude polarization during group discussions". Journal of Rersonality and Social Psychology (American Psychological Association) 68 (6): 1014–1029. doi:10.1037/0022-3518.104.22.1684. ISSN 0022-3514.
- Baldassarri & Bearman 2006
- Kelly 2007
- Lord, et al. 1979, p. 2100
- Lord, et al. 1979, pp. 2102, 2105-6.
- Lord, et al. 1979, pp. 2106-7.
- Lord, et al. 1979, pp. 2105-6.
- Lord, et al. 1979, p. 2101n1.
- Cooper, et al. 2004, pp. 252 ff.
- Mackie and Cooper 1984
- Baldassarri, Delia, and Bearman, Peter (2006). 'Dynamics of Political Polarization'. Working Paper No. 2006
- Cooper, Joel, Kelly, Kimberly A. and Weaver, Kimberlee (2004). 'Attitudes, Norms and Social Groups'. In Marilynn B. Brewer and Miles Hewstone, (eds.) Social Cognition. Oxford: Blackwell Publishing. ISBN 1-4051-1070-8, ISBN 978-1-4051-1070-9
- Fine, Cordelia (2006a). A Mind of its Own - How Your Brain Distorts and Deceives. W. W. Norton. ISBN 0-393-06213-9
- Fine, Cordelia (2006b). 'The Vain Brain'. An excerpt from Fine 2006a. In The Guardian, January 26, 2006.
- Fine, Cordelia (2007). 'Empathy' on Counterpoint, ABC Radio National. 1 October 2007
- Kelly, Thomas (2007). 'Disagreement, Dogmatism, and Belief Polarization', forthcoming in The Journal of Philosophy.]
- Lord, Charles, Ross, Lee, and Lepper, Mark (1979). 'Biased Assimilation and Attitude Polarization: The Effects of Prior Theories on Subsequently Considered Evidence',Journal of Personality and Social Psychology, 37 (11): 2098-2109.
- Mackie, D.M. and Cooper, J. (1984). 'Group polarization: The effects of group membership'. Journal of Personality and Social Psychology, 46: 575-585.
- Sunstein, Cass (2002). 'The Law of Group Polarization', The Journal of Political Philosophy, 10 (2): 175-195.