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How "default options" should be used in health risk assessment divides the risk analysis profession. Some argue that these should be "hard": set by policy, generally biased to be "health protective" and requiring a substantial body of evidence to replace by decision-specific alternatives. Others argue that they should be science-based, identified by consensus of the professional community, replaced by whatever decision-specific information may be available to the analyst. This paper shows that both positions have validity, and that both are incomplete. Each kind of construct has a useful role to play, but in different kinds of decisions. Because the two are different, we suggest giving them different names, "default option" being assigned to the policy-based construct, "inference option" (NRC, 1983) assigned to science-based assumptions, etc. We develop a theory that explains why these two different kinds of construct exist, and comment on some of the implications.

"Inference options" constitute an integral part of human health risk assessment, providing practitioners with consensus theories, models, or parameters that can be used to bridge knowledge gaps in specific analyses. Because human health risk assessment is both considered "scientific" and employs scientific reasoning, inference options must be treated as priors in an empirical-Bayesian inference process. Decision- or case-specific information modifies each prior according to the reliability of this information, with conflicts resolved by a scientific, weight-of-evidence process. Inference options are science-based "best estimates" and evolve through consensus within the professional community.

"Default options" constitute policy-derived components of particular kinds of decisions, serving as instructions to analysts. Such use is appropriate when many very similar, nontrivial decisions are to be made by a particular agency. In these decisions, which we suggest calling "recurrent," value-judgments are prescribed in advance, usually by legislation; generally only two decision options exist and the decisions usually turn on judgments made by experts. Authority to make these decisions is often delegated (sometimes tacitly) to permanent staff members who have the requisite expertise.

Policy-based default options exist in part because delegation of decision authority carries risks for organizations; those to whom it is delegated may unwittingly make decisions differently from senior officials, and may thus in some way harm the organization. Thus, delegation of authority is always conditioned by various forms of controls, including limits on the authority. We postulate that defaults serve as one means to control delegation risk. (They also simplify decision-making and make it more consistent.) These default options need to be tailored to the policy ends served, which generally means that some or all will be biased. They must be "hard," with "departure from" them requiring a high standard of evidence and also assurance that choosing an alternative will still satisfy the policy ends of the decision process in which these are a part. Default options need to be developed in the same way as any other policy-implementation practices, including deliberations that engage those who will be affected by the decisions. Such deliberations do not always take place.