It is reasonable to assume that the characteristics that define us are distributed across our population like a bell curve, which has a central bulge including about two-thirds of the people with the characteristic, and two "tails" on either side of the bulge that each include about one-sixth of them. A typical characteristic is influenced equally by our biology and experience, with each of its aspects (observable values, such as the color of one's eyes) best suited to survival and reproductive success in a particular type of environment. If the environment changes, experience may compensate faster than biology, which depends on generations of breeding under restricted conditions to manifest significant changes. To a high degree, each of us shares the same genetic makeup even though we manifest very specific aspects (values of characteristics), which as a practical matter means that over enough time and environments some of our descendants may have aspects like people in other parts of the population.
The greatest differences between people are between those at the extremes (the ends of the bell curve's tails), but the actual number of people involved is small: precisely one person for each extreme. At the beginning of civilization, each of those two people was one-millionth of the world's population. By 2013, each person was one of 7.1 billion people, the difference between them (the width of the bell curve) was one-third larger than at the beginning of civilization due to the increase in population, and each of the original one-millionths of the population had grown to the size of a town.
Each characteristic imposes its own scale on this overall population curve based on the range of possible aspects. That is, the width of the curve is measured as the difference between the highest and lowest values, and the center of the curve is the average of the two. The units will be different for different characteristics, though they all describe the same population. If the low or high value changes, then the curve will appear to either change its width or its position (sliding forward or backward), resulting in everyone changing – again, relative to the particular characteristic. If the changes allow for a larger population, then the population may grow; if the changes restrict the population, it may decrease.
For example, if consumption of a particular resource is the characteristic, then greater access to that resource will increase the maximum amount that can be consumed, widening the curve as long as someone continues to consume the minimum amount. If the minimum consumption is increased by people by distributing resources, the curve may then shrink in width and appear to have moved as a whole toward greater values, reflecting the fact that everyone is consuming an equal amount more than before access was increased. If access to the resource is reduced, however, and the minimum consumption is reduced, the curve may slide backwards; and if there is some critical minimum value of consumption below which people can't survive, the result will include a reduction in population.
For a critical characteristic – one whose changes affect population – it makes sense that people would resist changes that could decrease the population, or even be perceived to do so. Those who could experience any reduction in population first would be most resistant; but unfortunately, because they are likely to be on a tail of the bell curve (such as those at the lowest level of consumption in the example above) and have only a small number of people to help (people like them), they would be the least able to prevent it unless they could increase their own numbers to compensate.
The least vulnerable people, at the end of the other tail, would experience the narrowing of the curve associated with an increase in the minimum value, without a corresponding increase in the average, as a personal decrease similar to a sliding of the curve toward population loss, and instinctively resist it for that reason. If their resources were great enough to compensate for their lack of numbers, they might even be somewhat successful at doing so.
Given the inherent inertia in the bell curve's central bulge, it would be tempting for people to attribute a change to those on the tail associated with the direction of the change. For instance, increases in the average would be attributed to those on the leading edge of the curve, and decreases would be attributed to those on the trailing edge. To actually make a change, though, the people in the bulge would need to be convinced to actually make the change. During such a change, the bell shape of the curve might not be preserved; there are other curves that could describe the population until, in a more stable state, it settles into its new yet familiar form.