The curiously misunderstood role of evidence in designing new technology.

" Flying safely has not developed using experimental and control groups of air passengers and counting victims. " —Ülo Kristjuhan 75 R ECENTLY, I TOOK PART in a radio interview together with the demographer Jay Olshansky. Jay and I have been friends for over a decade, and we have done this before: indeed , he and I have for most of that period been among the most frequently appearing academic gerontologists in mainstream media. The interview was trailed as being about the desirability of combating aging, 1–3 a topic on which Jay and I wholeheartedly agree. In practice, however, the interviewers chose to focus heavily on the feasibility of doing so. On this point, Jay and I are sharply at loggerheads. While he believes that there may be some modest progress in slowing " the basic process of aging " in the coming few decades, he is adamant that there is no chance of success in using re-generative medicine to comprehensively reverse aging. I take the opposite view on both counts: I consider that appreciably slowing the accumulation of molecular and cellular damage that underlies aging is an extremely daunting challenge, since that damage is created as intrinsic side effects of metabolism , whereas repairing pre-existing damage is far more realistic. So far, so tedious. Scientists often disagree about the interpretation of data, and in this particular case the views that Jay and I expressed were not even novel. 4,5 But the situation becomes more worthy of analysis when one delves into the reasoning we used in justifying our predictions. Both of us appeal, in support of our conclusions, to the existence or absence of evidence. Specifically, Jay avers that there can be no evidence to support the efficacy of regenerative medicine against aging other than the successful extension of a mam-mal's healthy life using such techniques, and he views my claims to the contrary (specifically, claims that we already have evidence that such an approach will probably work) as unscientific. It is tempting to dismiss Jay's views here on the basis that he is " only a demographer, " not versed in the evaluation of experimental data, but that would be wide of the mark: not only because it is ad hominem, but also because highly distinguished experimentalists share his view. For example, the respected gerontologist Robin Holliday published an article recently with the uncompromising title " The extreme …