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overloaded by weaponry and ammunition at the expense of their durability. Such machines could last much longer in a protected environment if the many dangerous fighting de- vices designed for combat were removed. The same is true for living organisms; loss of some functions through intro- duced mutations or other interventions often leads to in- creased species longevity in a protected environment. This observation is sometimes interpreted as proof that aging is a programmed process, while in fact it simply means that or- ganisms were selected by Nature for survival in a wild hostile environment, rather than for longevity in protected laboratory conditions.

nize more joint conferences of demographers with policy- makers and other opinion-formers, and to arrange more public meetings and discussions between demographers and influential people. Such a strategic outreach program could become successful with the support of foundations led by wealthy visionaries such as Peter Thiel, Bill Gates, or Larry Ellison, for example.

It might also be helpful to strengthen the demographic component in current antiaging research programs, such as the Science Against Aging program, which was recently developed by Science for Life Extension, an international foundation.


Recently you developed novel software to describe the de- mographic consequences of therapies that genuinely re- verse, rather than merely retard the advance of biological age (i.e., the life-long accumulation of the ‘‘damage’’ that causes age-related ill-health when it becomes sufficiently abundant). Were you surprised at the predictions that this scenario yielded?

Yes, we were surprised by some of our own findings. For example, consider the ‘‘worst-case’’ scenario (for overpopu- lation), that being physical immortality, or no deaths at all. What would happen to population numbers in that case? Common sense and intuition would predict a demographic catastrophe if immortal people were to continue to repro- duce; that is what we initially believed too. However, a deeper mathematical analysis leads to paradoxical results. If parents produce less than two children on average, so that each next generation is smaller by some common ratio (R < 1), then even if everybody were immortal, the size of the population over time would not be infinite; instead it would be just 1=(1 R) times larger than the initial population. For example, one-child reproduction practices (R ¼ 0.5) would only lead to a doubling of the total immortal population, because 1=(1–0.5) ¼ 2. In other words, a population of im- mortal reproducing organisms can grow indefinitely in time, but not necessarily indefinitely in size, because asymptotic growth is possible.32 The beauty of this finding is that it does not require any complex calculations and questionable as- sumptions, but rather follows directly from the calculus and the fact that infinite geometric series converge when the absolute value of the common ratio (R) is less than one. The startling conclusion is that fears of overpopulation based on lay common sense and uneducated intuition are, in fact, grossly exaggerated. It came as a great surprise to us that just a bit of clear thinking and simple mathematics can make such a difference in resolving a problem.

Demographic predictions are of immense importance in de- termining economic and social policy, in view of the impact of factors such as dependency ratio on the relative economic viability of different policies. Thus, it seems essential that policy-makers and other opinion-formers take demogra- phers’ work seriously. What can be done to encourage in- fluential people to understand the importance of such predictions and to act on them?

To increase the impact of demographic predictions on economic and social policy, it is important to have a very active and persistent strategic outreach program. In other words, we need to stimulate demographers to publish more reader-friendly articles in high-profile publications, to orga-

What factors in the industrialized world do you think most strongly influence the following: (1) The number of children women have; and (2) the average age at which women have their first child? How do you think these factors would be altered by the advent of therapies that combat aging, includ- ing menopause?

We believe that the key variable most strongly influencing fertility in the industrialized world is the ‘‘opportunity cost’’ of having a child. This is the cost of childbearing in terms of time and resources, and, most importantly, the intensity of competition between this choice and other opportunities in life. For example, a demanding job could prevent some career-oriented women from having a child before they have established themselves. The risk of losing a partner could be another opportunity cost, particularly for unmarried couples, cohabitating without a strong commitment. The desire to have a flexible lifestyle (primarily at younger ages), or con- cerns about the health consequences of childbearing (par- ticularly at older ages) represent other examples of the perceived opportunity costs of having a child.

With the advent of therapies that combat aging, including menopause, the pressure on women to have a child ‘‘before it is too late’’ may decrease. However, we have to make sure that with these new therapies the children born to older par- ents will be as healthy as those born to younger parents. Our own studies, as well as the abundant biomedical literature, indicate that currently both advanced paternal and maternal age at the time of conceiving a child can have significant negative effects on a child’s health and longevity. Therefore, at this time, it is better to avoid delayed parenting. 31,46–48


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