Introduction
In arguing for increased research funding, attention is often drawn to the finding that schizophrenia accounts for 2.3% of the total burden of disease (disability adjusted life years, DALYs) in established market economies (Murray and Lopez, 1996). How is it that, despite 1.4 - 2.8% of national health care being devoted to the direct costs of schizophrenia, the burden of disability is still so high? What would the burden of schizophrenia be if funds were unlimited and optimal treatments (medication, psychosocial interventions, service mix, etc.) were delivered consistently?

Most commentators would concede that the burden would still be inevitable. In other words, a substantial proportion of the DALYs associated with schizophrenia are ‘unavertable’ in terms of secondary and tertiary prevention. An alternative, and more ambitious, approach to averting DALYs is to reduce the incidence of a disorder. This section will discuss issues related to primary prevention in general and then speculate on directions for future research related to schizophrenia.

The science of prevention
In its simplest form, primary prevention aims to reduce the incidence of a disease.

Prevention strategies can be directed at different target populations (Gordon, 1983; Mrazek and Haggerty, 1994): (i) universal preventive interventions are aimed at the general population regardless of risk status/susceptibility status; (ii) selective preventive interventions target particular population subgroups, who may be more susceptible to a disorder but who are still symptom free; (iii) indicated prevention is targeted at individuals who have the early features or subclinical manifestations of a disorder.

Universal interventions have strengths and weakness that relate to the features of both the exposure (the risk-modifying variable, be it genetic, epigenetic or an interaction between the two) and the disorder. This approach alleviates the need to identify a minority of individuals who are ‘high-risk’  -  the focus of much current research in the prevention of schizophrenia. If we can identify such individuals, and if we can reduce their risk, then this is a highly desirable goal. However, if we cannot identify high-risk individuals, we need to consider alternative strategies.

Geoffrey Rose (1992) has emphasized that population-based interventions are best suited to risks that are distributed throughout the population, albeit not in equal measure. Those at high risk of disease, seemingly an obvious target for preventative action, may in fact be relatively rare. Those at medium risk may be more common and, therefore, may account for a much higher proportion of disease.

For example, if a large proportion of the community is exposed to a small risk, then population-based interventions may avert more illness (greater number of cases prevented) than interventions based on the rare, high-risk individuals. Rose (1992) then introduced the concept of the ‘prevention paradox’  -  a preventive measure that brings large benefits to the community but which offers little to the majority who are, themselves, at low risk. Indeed, the intervention may mean that such individuals have to give up something; hence the paradox. For example, many population-based interventions (e.g.  vaccination,  wearing a seatbelt)  bring little direct benefit to the individual, but individuals are willing to accept them because they cause little inconvenience. Inconvenience is weighed against the frequency of the undesirable outcome and its severity.

John McGrath
Queensland Centre for Schizophrenia Research, Wolston Park Hospital, Wacol, Australia

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