4.1. Diagnosing the broken leg problem

The broken leg problem arises when a reasoning strategy that has been

proven reliable on a particular class of problems is applied to a problem

that is thought (rightly or wrongly) to be outside the range of problems for

which the strategy is known to be reliable. For example, the VRAG (Violence

Risk Appraisal Guide) test for violent recidivism was developed

primarily as the result of research done on a population of violent Canadian

psychiatric patients at the Oak Ridge Division of the Penetanguishene

Mental Health Care Center (Quinsey et al. 1998, xi). When using the VRAG,

one might reasonably wonder whether it is reliable on different subpopulations,

such as non-psychiatric patients or criminals in the U.S. (In both

cases, it is.) One way to pose the broken leg problem is to ask: Under what

conditions is it reasonable to defect from a reasoning strategy that has been

shown to be reliable for a particular class of problems?

The broken leg problem is a serious and pressing issue for any theory

that embraces the findings of Ameliorative Psychology. On the one hand,

it is absurd to suppose that one should never defect from a successful SPR.

On the other hand, people have a hard time avoiding the temptations of

defection. And excessive defection undermines reliability. After all, whenever

an SPR is more reliable than human judgment and the expert and the

SPR disagree, the SPR is more likely to be correct. In the long run, reliability

is reduced if one insists upon consistently replacing more reliable

reasoning strategies with less reliable reasoning strategies.

This intuitively powerful argument has been confirmed a number of

times in the laboratory. There are a number of studies in which subjects are given SPRs and then are permitted to selectively defect from them (i.e.,

override them), sometimes after having been told that the SPR by itself has

been shown to be more reliable than experts. Typically, subjects find more

broken leg examples than there really are. As a result, the experts predict

less reliably than they would have if they’d just used the SPR (Goldberg

1968, Sawyer 1966, Leli and Filskov 1984). (Interestingly, it doesn’t usually

seem to matter whether the subjects are experts or not.) Selective defection

strategies generally have a poor track record (except when the defectors

have expertise in a theory with significant predictive success).

The broken leg problem and the failure of selective defection strategies

suggest that any epistemic theory that hopes to take full advantage of

the prescriptive power of Ameliorative Psychology must do more than put

forward and recommend reliable SPRs. It must include a psychological

theory of human judgment that can anticipate the difficulties we will have

implementing the best available reasoning strategies. It is an unfortunate

fact about humans that we are too often tempted to defect from successful

SPRs. A normative theory with prescriptive force needs to predict the ways

in which we are likely to deviate from excellent reasoning and perhaps

provide methods of preventing such unfortunate deviations. Of course, we

don’t pretend to have such a theory; accordingly, our discussion of this

matter will be tentative and programmatic. But we take this to be a prime

example of how a reason-guiding epistemology will essentially depend on,

and be informed by, a mature empirical psychology.

The broken leg problem arises when a reasoning strategy that has been

proven reliable on a particular class of problems is applied to a problem

that is thought (rightly or wrongly) to be outside the range of problems for

which the strategy is known to be reliable. For example, the VRAG (Violence

Risk Appraisal Guide) test for violent recidivism was developed

primarily as the result of research done on a population of violent Canadian

psychiatric patients at the Oak Ridge Division of the Penetanguishene

Mental Health Care Center (Quinsey et al. 1998, xi). When using the VRAG,

one might reasonably wonder whether it is reliable on different subpopulations,

such as non-psychiatric patients or criminals in the U.S. (In both

cases, it is.) One way to pose the broken leg problem is to ask: Under what

conditions is it reasonable to defect from a reasoning strategy that has been

shown to be reliable for a particular class of problems?

The broken leg problem is a serious and pressing issue for any theory

that embraces the findings of Ameliorative Psychology. On the one hand,

it is absurd to suppose that one should never defect from a successful SPR.

On the other hand, people have a hard time avoiding the temptations of

defection. And excessive defection undermines reliability. After all, whenever

an SPR is more reliable than human judgment and the expert and the

SPR disagree, the SPR is more likely to be correct. In the long run, reliability

is reduced if one insists upon consistently replacing more reliable

reasoning strategies with less reliable reasoning strategies.

This intuitively powerful argument has been confirmed a number of

times in the laboratory. There are a number of studies in which subjects are given SPRs and then are permitted to selectively defect from them (i.e.,

override them), sometimes after having been told that the SPR by itself has

been shown to be more reliable than experts. Typically, subjects find more

broken leg examples than there really are. As a result, the experts predict

less reliably than they would have if they’d just used the SPR (Goldberg

1968, Sawyer 1966, Leli and Filskov 1984). (Interestingly, it doesn’t usually

seem to matter whether the subjects are experts or not.) Selective defection

strategies generally have a poor track record (except when the defectors

have expertise in a theory with significant predictive success).

The broken leg problem and the failure of selective defection strategies

suggest that any epistemic theory that hopes to take full advantage of

the prescriptive power of Ameliorative Psychology must do more than put

forward and recommend reliable SPRs. It must include a psychological

theory of human judgment that can anticipate the difficulties we will have

implementing the best available reasoning strategies. It is an unfortunate

fact about humans that we are too often tempted to defect from successful

SPRs. A normative theory with prescriptive force needs to predict the ways

in which we are likely to deviate from excellent reasoning and perhaps

provide methods of preventing such unfortunate deviations. Of course, we

don’t pretend to have such a theory; accordingly, our discussion of this

matter will be tentative and programmatic. But we take this to be a prime

example of how a reason-guiding epistemology will essentially depend on,

and be informed by, a mature empirical psychology.