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The evolution heuristic instructs us to consider, for an apparently attractive enhancement, why we have not already evolved the intended trait if it is really such a good idea. We called this question the evolutionary optimality challenge, and we have described three broad categories of possible answers and given some examples of particular enhancements for which it is possible to meet the EOC and which, therefore, seem comparatively promising as intervention targets that may be feasible in the relatively near term and which may have on balance beneficial effects.

In general, when we pose the EOC for some particular proposed enhancement, we might discover one of several things:

  • 1. Current ignorance prevents us from forming any plausible idea about the evolutionary factors at play.
  • 2. We come up with a plausible idea about the relevant evolutionary factors, and this reveals that the proposed modification would likely not be a net benefit.
  • 3. We come up with a plausible idea about the relevant evolutionary factors, and this reveals why we would not already have evolved to have the enhanced capacity even if it would be a net benefit.
  • 4. We come up with several plausible but mutually inconsistent ideas about the relevant evolutionary factors.

The first possibility means that we have no clear idea about why, from a phylogenetic perspective, the trait that is the target of the proposed enhancement is the way it is. This should give us pause. If we do not understand why a very complex evolved system has a certain property, there is a considerable risk that something will go wrong if we try to modify it. The case might be one of those where nature does know best. Like an overambitious tinkerer with merely superficial understanding of what he is doing while he is making changes to the design of a master craftsman, the potential for damage is considerable, and the chances of producing an all-things- considered improvement are small.

We are not claiming that it is always inadvisable to try an intervention when we have no adequate understanding of the subsystem we intend to enhance. We might have other sources of evidence that afford us sufficient assurance that the intervention will work and will not cause unacceptable side effects, even without understanding the evolutionary functions involved. For example, we might have used the intervention many times before and found that it works well. Alternatively, we might have evidence from a closely analogous subsystem, such as an animal model, that suggests that the intervention should work in humans too. In such cases, the evolution heuristic delivers only a weak recommendation: that absent any good answer to the EOC, we should proceed only with great caution. In particular, we should be alert to the possibility that the proposed intervention will turn out to have significant (but perhaps subtle) side effects.

The second possibility is that we succeed in developing a plausible understanding of the pertinent evolutionary factors, and, having done so, we find our initial hopes about the proposed modification undermined. None of the three categories we have described yields a satisfactory answer to the EOC: the relevant tradeoffs have not changed since the EOC, there is no relevant value discordance, and no evolutionary restriction would have prevented the modification from already having evolved by now. In this case, we have strong reason for thinking that the enhancement intervention will fail or backfire. If we proceed, the wisdom of nature will bite us.

The fourth possibility is that we come up with two or more plausible but incompatible evolutionary accounts of the evolutionary factors at play. In this case, we can consider the implications of each of the different evolutionary accounts separately according to the above criteria. If all yield green lights, we are encouraged to proceed. If some of the evolutionary accounts yield green lights but others yield red lights, then we face a situation of uncertainty. We can use standard decision theory to determine how to proceed—we can take a gamble if we feel that the balance of probabilities sufficiently favors the green lights; if not, we can attempt to acquire more information in order to reduce the uncertainty or forgo the potential enhancement and try something else.

The evolution heuristic is not a rival method to the more obvious way of determining whether some enhancement intervention works: testing it in well-designed clinical trials. Instead, the heuristic is complementary. It helps us ask some useful questions. By posing the EOC and carefully searching for and evaluating possible answers in each of the three categories we described, we can (a) identify promising candidate enhancement interventions, to be explored further in laboratory and clinical studies, and (b) better evaluate the likelihood that some intervention which has shown seemingly positive results in clinical studies will actually work as advertised and will not have unacceptable side effects of a hidden, subtle, or long-term nature.

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