This basic logic is represented graphically by a diminishing returnscurve: each additional unit is worth less and less [Boone Fig. 10.5(b)]On the other hand, one can imagine why the bottom of the curve might show accelerating returns (each additional unit worth morethan last); if you've gone long without food, the first bite whets your appetite and leaves you hungrier than before! [Boone Fig. 10.5(c)]Keeping our example of yield as food income but shifting our value currency from satisfaction or utility to fitness, consider why the same general logic might apply: an adult human of average size limited to 100 kcal a day will soon perish, but at 500 kcal/day may be able to hang on a few weeks; by 1500 kcal/day will be hungry but surviving; by 2000 kcal/day will have enough to be active (and resist diseases and other insults), at 2500 kcal/day has enough to reproduce, and beyond that may gain little fitness benefit (though even 4000 kcal/day may offer some marginal fitness gain in a
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