There are three main approaches to evaluating whether a particular system of justice is fair: the outcomes model, the balancing model, and the participation model.
Outcomes model The idea of the outcomes model of procedural justice is that the fairness of process depends on the procedure producing correct outcomes. For example, if the procedure is a criminal trial, then the correct outcome would be conviction of the guilty and exonerating the innocent. If the procedure were a legislative process, then the procedure would be fair to the extent that it produced good legislation and unfair to the extent that it produced bad legislation. This has many limitations. Principally, if two procedures produced equivalent outcomes, then they are equally just according to this model. However, as the next two sections explain, there are other features about a procedure that make it just or unjust. For example, many would argue that a benevolent dictatorship is not (as) just as a democratic state (even if they have similar outcomes).
Balancing model Some procedures are costly. The idea of the balancing model is that a fair procedure is one which reflects a fair balance between the costs of the procedure and the benefits that it produces. Thus, the balancing approach to procedural fairness might in some circumstances be prepared to tolerate or accept false positive verdicts in order to avoid unwanted costs (political) associated with the administration of criminal process.
Ronald Dworkin argued that a properly balanced procedure is one that values peoples' rights and treats persons equally.
The participation model The idea of the participation model is that a fair procedure is one that affords those who are affected by an opportunity to participate in the making of the decision. In the context of a trial, for example, the participation model would require that the defendant be afforded an opportunity to be present at the trial, to put on evidence, cross examination witnesses, and so forth.
Group engagement model Models have also been proposed to understand the psychological basis of justice. One of the more recent of these models is the group engagement model. The group engagement model (GEM), devised by
Tom R. Tyler and Steven L. Blader, incorporates past psychological theories to explain the underlying psychological processes of procedural justice. Based on
social identity theory and relational models of procedural justice, this model suggests that a group's procedural justice process influences members' identification with the group, which in turn influences their type of engagement within the group. According to the model, group engagement is seen as either mandatory or discretionary behavior. Mandatory behavior is defined by Tyler and Blader as behavior that is required by the group and thus is motivated by incentives and sanctions. Conversely, discretionary behavior is motivated by internal values and is seen as more cooperative and therefore ideal within a group. Depending on the procedural justice processes of the group, the social identity of the members will be influenced accordingly and different values will be emphasised. The more a member agrees with the type of procedural justice employed, the more they will identify with their group. This increased identification results in the internalization of the group's values and attitudes for the group member. This creates a circular relationship as the group's procedural justice processes will affect group members' levels of identification and, as a consequence, this level and type of identification will affect their own values of what is fair and unfair. This, in turn, will then affect how the individuals will engage with their group, with higher identification leading to discretionary and more desirable behavior. == Due process and natural justice ==