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inexpensive blood test, shortly after birth, accurately identifies infants who lack
the enzyme that metabolizes certain proteins and are at risk of mental retardation
due to phenylketonuria (PKU). Such children can be treated successfully with
special diets low in the amino acids that give rise to phenyls.
Unfortunately, most unwanted phenomena are not much like PKU. No one
factor accurately predicts them. Rather, a number of factors have predictive power,
with risk increasing as risk factors multiply or are "bundled." Even so, the accuracy
of prediction often is not particularly high. This results in poor targeting and conse-
quent inefficiencies in prevention programs even when the interventions work
as intended. But the example of PKU is instructive in one important respect:
Although the problem in cases of PKU is the lack of an enzyme, the effective pre-
vention measure does not replace the enzyme but modifies the child's diet instead.
We will suggest that in the case of homelessness, as well, the solution may not
always be isomorphic with the problem.
Prevention programs are of three ideal types (Mrazek & Haggerty, 1994, fol-
lowing Gordon, 1983). Universal prevention programs are available to the entire
population, although they are sometimes targeted at people at a particular stage of
life. Such programs may be narrow and inexpensive (childhood immunizations to
prevent measles) or inclusive and expensive (old-age pensions to prevent poverty
among the elderly; water treatment facilities to prevent water-borne disease). Pre-
vention programs may strengthen individuals (a measles vaccine) or change the
environment (water treatment). Selected prevention programs are aimed at people
at risk because of membership in some group. No individual screening is required
for participation (an educational program aimed at data entry clerks at risk of repet-
itive motion injuries). Indicated prevention programs are directed to people at risk
because of some individual characteristic or constellation of characteristics, deter-
mined by individual-level screening (programs to mitigate the consequences of
genetic diseases).
Selected and indicated strategies may be more efficient than universal mea-
sures when it is easy to identify and deliver interventions to groups of people or
individuals at risk for a particular condition. The efficiency of targeting is thus of
fundamental importance to the design of prevention programs, and the costs of
targeting must be compared with the costs of offering programs more broadly or
allowing people to select themselves for universal programs attractive only to
those with high levels of perceived need.
The Prevention of Homelessness Revisited
Ideal types are heuristic devices, of course, and distinctions among types of prevention are often
fuzzy. For instance, people discharged from mental hospitals comprise a group at risk of homelessness
but also have the individual risk factor of prior mental hospitalization. A "universal" housing program
may come with strings attached that make it attractive only to those who are poor (an individual risk