Many theories attempt to explain why non-compliant behaviour occurs and how it can be discouraged. Psychological theories of compliance with rules and social norms often assume that the decision-making processes of rule breakers differ from those of other people. Cognitive theories of compliance argue that these behaviours stem from differences in personal moral development (e.g. Goslin, 1969; Tapp & Levine, 1977; Kohlberg, 1984). Another important group, the social learning theories (e.g. Sutherland, 1947; Burgess & Akers, 1966; Akers, 1985), suggest that individuals' decision-making processes are conditioned by interactions with their environment.
By contrast, sociological and economic theories of compliance assume that the decision-making process in rule breakers is not fundamentally different from that in other people. Normative theories argue that an individual's perceptions of the legitimacy and fairness of rules are crucial to decision making (Tyler, 1990). Instrumental theories, on the other hand, hold that acts of non-compliance occur because the benefits anticipated by the decision maker outweigh the costs (Becker, 1968).
The study of optimal enforcement has largely focussed on instrumental approaches, using economic models to answer the question of how best to modify individual incentives in favour of compliance. An individual's supply of offences may be modelled as a decreasing function of two factors of enforcement: the probability of an act of non-compliance being detected and punished and the severity of punishment that results (Becker, 1968). The process of detection is inherently costly, requiring law enforcers to be paid and equipped, whereas punishments may take the form of fines (assumed to be a costless transaction). This suggests that the optimal enforcement strategy is to reduce the amount of costly monitoring while increasing the size of penalty, thereby maintaining offences at an acceptable level with lower enforcement costs.
There are, however, several reasons why severe penalties may be undesirable. Extensions to Becker's model suggest that if sanctions are socially costly (Kaplow, 1990) or if corruption is present (Becker & Stigler, 1974), the optimal fine level may not be the highest possible. Similarly, if individuals are risk averse (Polinsky & Shavell, 1979), are imperfectly informed about their probability of being caught (Bebchuk & Kaplow, 1992), respond to penalties by trying to avoid detection (Malik, 1990), or vary in their wealth (Polinsky & Shavell, 1991) the optimal level of fines may be reduced. Severe penalties are also morally questionable and can lead to an increase in serious crimes relative to less damaging offences due to the loss of marginal deterrence (Stigler, 1970).
A large number of studies have attempted to empirically test the deterrence effect of enforcement measures upon crime rates in developed countries, with mixed results (see Cameron, 1988 for a review). Ehrlich (1996) argues that there is such an effect and that the probability of detection may be more influential than severity of punishment. However, issues such as the use of data at different levels of aggregation, uncertainty about the level of private protection and the difficulty in separating the influence of deterrence and incapacitation leave many studies open to criticism (Cameron, 1988; Ehrlich, 1996).
Early bioeconomic models of NRM assumed enforcement was costless and produced perfect compliance (e.g. Clark, 1990). The implications of imperfect enforcement in NRM were first explored in commercial fisheries. In quota-restricted single-species fisheries, for example, enforcement costs may be modelled as an increasing function of the stock size and the legal quota. Consequently, the larger the desired stock size (above the open-access equilibrium) the greater the necessary expenditure on enforcement (Sutinen & Andersen, 1985). More generally, the optimal level of enforcement is attained when the marginal cost of enforcement is equal to its marginal benefit (Becker, 1968; Sutinen & Andersen, 1985; Hallwood, 2005). Other models of fisheries enforcement have considered differences between input controls (e.g. time at sea, equipment) and output controls (e.g. landing quotas) (Mazany, Charles & Cross, 1989), and shown that avoidance behaviour affects the socially optimal level of enforcement (Anderson & Lee, 1986; Anderson, 1987).
Several studies have attempted to empirically test the predictions of fisheries enforcement models. Survey data from the Massachusetts lobster fishery show an increasing rate of compliance as the perceived probability of being caught increases (Sutinen & Gauvin, 1989). Similarly, data from Quebec fisheries show a greater influence of the probability of detection than severity of punishment on offences (Furlong, 1991). Data from federally managed US groundfish fisheries, on the other hand, suggest that a decline in compliance from 1982 to 1988 was best explained by poor stock conditions and high market prices, with enforcement having a negligible effect (Sutinen, Rieser & Gauvin, 1990).
The effects of the design of enforcement on poaching decisions have also been explored. Using a model of multi-species bushmeat hunting as a component of the household economy, measures targeting bushmeat sales were shown to be more effective than those targeting hunting directly (Damania, Milner-Gulland & Crookes, 2005). The benefits for different hunted species are complicated by technology switching (e.g. between snaring and gun hunting), however, and are therefore ambiguous. Clayton, Keeling & Milner-Gulland (1997) investigated economic deterrents to hunting two wild pig species in Indonesia, only one of which can be legally hunted. A fine on market dealers for selling the illegal species was shown to be most effective, and more equitable than other approaches considered because it does not affect the welfare of individuals who hunt the legal species.
Morality, equity and justice
Alternative models of compliance with regulations emphasize the role of normative factors, such as moral obligation and perceptions of fairness and justice. Normative factors have been incorporated into economic frameworks by assuming that an individual's utility is increased by performing actions that are socially acceptable or beneficial (Sutinen & Kuperan, 1999; Nielsen, 2003). The perceived legitimacy of rules, related to both the fairness and efficiency of the regulatory process and the justice and effectiveness of its outcomes, also affects their acceptance by resource users (Hønneland, 1999; Sutinen & Kuperan, 1999).
In Norway and Newfoundland, some small fisheries achieve high levels of compliance despite low levels of formal enforcement. Gezelius (2002, 2004) suggests that this results from informal sanctions based upon collective moral judgements. Non-compliant individuals are subjected to social opprobrium if their actions are perceived to confer unfair advantages or to be carried out for monetary gain, but not if they are necessary to secure an adequate basic income.
Quantitative empirical evidence on the influence of normative influences on compliance is, however, weak. Nielsen & Mathiesen (2003) identified factors which have a major influence on compliance in small Danish fisheries. The most important factors were instrumental: economic gains and deterrence measures, but normative considerations such as the fairness of rules were also represented. Hatcher et al. (2000) similarly reported a significant positive relationship between perceptions of fairness and participation and levels of compliance in the UK fishery, but Hatcher & Gordon (2005) failed to reproduce this result, finding instead that economic incentives dominate.