Within the oyster grows a hidden pearl / From the deep longings of the boundless sea. -O.K.

Variations in population projections illustrate differences in assumptions about the working of negative and positive feedback in human reproductive behavior. At the beginning of the nineteenth century, Malthus assumed that human communities always bred up to their survival potential. In other words, positive feedback would lead to growth curbed eventually by mass famine or other disaster, a course only to be endlessly repeated through history. This approach is modeled for our future in the first two trendlines in the chart above. The first assumes that humanity will keep testing the limits, with alternations between short periods of growth and decline. The Malthusian disaster assumes that the results of too rapid growth can lead to an irreversible spiral down, with "positive" feedback in this case leading to an end of humanity — or at least of developed society.

We have accumulated a great deal of evidence that at least in our era human beings have more control than this, that negative feedbacks on growth have become much more complex than simple survival. This assumes that the pluses of childbearing will be balanced by the minuses in such a way that a rough equilibrium is achieved. Two different balanced adaptive futures are suggested, with the later assuming an eventual victory for the "less is more" approach of some environmentalists.

The lowest line assumes that a kind of Malthusianism in reverse will occur. Adaptive behavior in this case picks up steam through a positive feedback process that eventually makes children seem little more than unnecessary interference in advanced human civilization. Once the people of the world as a whole get the idea that a satisfying life does not require children, then reproduction falls out of fashion, and a positive feedback process causes a spiral down of population. This appears to have happened already in parts of Europe and Japan, but we have little idea how far the new fashion will spread. Unlike the original Malthusian model, the projected decline will be due to too much cultural control over reproduction rather than too little (assuming we want the species to survive).

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