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Rational ignorance occurs when the cost of educating oneself on an issue exceeds the potential benefit that the knowledge would provide.
Ignorance about an issue is said to be "rational" when the cost of educating oneself about the issue sufficiently to make an informed decision can outweigh any potential benefit one could reasonably expect to gain from that decision, and so it would be irrational to waste time doing so. This has consequences for the quality of decisions made by large numbers of people, such as general elections, where the probability of any one vote changing the outcome is very small.
The term is most often found in economics, particularly public choice theory, but also used in other disciplines which study rationality and choice, including philosophy (epistemology) and game theory.
Consider an employer attempting to choose between two candidates offering to complete a task at the cost of $10 / hour. The length of time needed to complete the task may be longer or shorter depending on the skill of the person performing the task, so it is in the employer's best interests to find the fastest worker possible. Assume that the cost of another day of interviewing the candidates is $100. If the employer had deduced from the interviews so far that both candidates would complete the task in somewhere between 195 and 205 hours, it would be in the employer's best interests to choose one or the other by some easily-applied metric (for example, flipping a coin) rather than spend the $100 on determining the better candidate, saving at most $100 in labor.
In many cases, the decision may be made on the basis of heuristics; a simple decision model which may not be completely accurate. For example, in deciding which brand of prepared food is most nutritious, a shopper might simply choose the one with (for example) the lowest amount of sugar, rather than conducting a research study of all the positive and negative factors in nutrition.
In marketing 
Marketers can take advantage of rational ignorance by increasing the complexity of a decision. If the difference in value between a quality product and a poor product is less than the cost to perform the research necessary to differentiate between them, then it is more rational for a consumer to just take his chances on whichever of the two is more convenient and available. Thus, it is in the interest of the producer of a lower value product to proliferate features, options, and package combinations until the average shopper finds it too much trouble to make an informed decision.
In politics 
This also works for politics. By increasing the number of issues that a person needs to consider in order to make a rational decision between candidates, they can level the playing field by encouraging single-issue voting or party-line voting.
Another, more nuanced, political application involves a voter's identification with a political party, much like the adoption of a favorite movie critic. Based on prior experience a reasonably responsible voter will find politicians or a political party that will draw conclusions similar to their own conclusions when the voter had the time to do the analysis.
Much of the empirical support for the idea of rational ignorance was drawn from studies of voter apathy, which reached particularly strong conclusions in the 1950s. However, apathy appeared to decline sharply in the 1960s as concern about issues such as the Vietnam War mounted, and political polarization increased. This suggests that voters' interest in political information increases with the importance of political choices.
Additionally, rational ignorance is scrutinized for its broadening effect on the decisions that individuals make in different matters. The investment of time and energy on the specified subject has ramifications on other decision areas. Individuals sometimes rationally ignore this when unconsciously assessing the investment cost versus payout. The external benefits are therefore not adequately taken into account.
See also 
- Campbell, A., Converse, P., Miller, W. and Stokes. D. (1960), 'The American Voter', Wiley, N.Y.
- Nie, N., Verba, S. and Petrocik, J. (1976), 'The Changing American Voter', Harvard University Press, Cambridge, Mass.
- Rational Ignorance vs. Rational Irrationality
- Rational Irrationality: Some Economics of Self-Management
- "Would Rational Voters Acquire Costly Information?" by Cesar Martinelli, Journal of Economic Theory, Vol. 129, Issue 1, July 2006, pp. 225-251 (subscription required)
- "Rational Ignorance and Voting Behavior" by Cesar Martinelli, International Journal of Game Theory. Vol. 35, Issue 3, February 2007, pp. 315-335 (subscription required)