Psychology today explains human behavior based on the interactions of nature and culture. These powerful but very different sets of forces work independently and sometimes together to produce the forms of human social life we see today.
Sometimes, however, nature and culture are at cross purposes. Let me engage in the risky business of forecasting the distant future. Into the next century, and possibly even by the second half of this century, I see a huge looming conflict between nature and culture over the issue of population.
Culture pushes for more population. There are several deeply rooted reasons for this, indeed rooted in the very essence of culture. A culture is a giant social system - in several crucial ways, the larger, the better.
First, culture is about sharing information. The more people who produce information and share it, the faster progress will be made. The pace of scientific progress and cultural change has accelerated over the last couple centuries, partly because of the increase in the number of people who are alive. That is, the pace of scientific progress has picked up partly because there are more scientists doing research. (Improved communication is also a contributing factor, so scientists can benefit from each other's work.)
Second, a culture is an economic system. Each person is an economic agent. Here again, the larger the system, the better it works. Big cities are stronger engines of economic growth and progress than small villages. Big companies are more efficient and work better than small ones. (When big stores or companies move into an area, the small ones often go bust; rarely does it work the other way around.) This is why globalization is relentless and inevitable: Integrating the whole world into one economic system will make a stronger, more effective system (and will most people better off), as compared to keeping lots of separate little economic systems than cannot trade and interact.
There are also historical reasons that cultures want more people. Throughout history (and prehistory), most groups have competed against other groups. In general the larger groups have won, whether on the battlefield or in the marketplace. Even today, as the world faces overcrowding and the rising costs of overpopulation, most countries still do all they can to promote population growth, such as by shoveling money to people to reward them for having children.
What about nature? Reproduction is the natural impulse, and natural selection has favored traits that promote reproduction. Thus, each organism has tendencies and impulses that promote making babies. To keep the population from escalating beyond the capacity of the environment to support it, nature has relied on some cruel checks and balances, including predation and disease. If there were too many organisms to support, many simply starved.
Thus, for most of human history, nature and culture conspired together to increase the population, and growth was restrained by nature's darker side. Today, however, we begin to see that the natural restraints on population are slowly being defeated by culture. Advances in public health and medicine have sharply reduced death by many diseases, especially early in life. Food is produced and distributed better, and there is realistic talk of ending hunger in the world. Predators who eat humans have generally been defeated. In the United States, as I understand, the animal that causes the most human deaths is now the deer. Deer do not hunt and kill humans; rather, several dozen times a year, deer wander into roads where automobiles strike them, and the collision sometimes kills the human occupants of the car.
The remarkable success of many cultural advances at reducing human death has enabled the human population to skyrocket. The twentieth century slowed this growth with a series of astonishing bloodbaths, such as the Chinese and Soviet purges and the world wars, but it now seems possible to hope that the 21st century will be mostly spared such large-scale horrors.
Where will this lead? Some seem to think that the world population can continue growing indefinitely. I think we are already bumping up against some limits, though they are disguised. If so, then some time in the next hundred years or so, we may reach a crisis point, at which there are so many people that the planet cannot support us. In trying to accommodate so many people, we will do irreversible damage to it. Ultimately this might lead to human extinction. It will certainly lead to many changes for the worse, even if our species does survive.
Many of the problems that face modern societies have overpopulation as the silent partner. Consider the fuel crisis. We blame the oil companies or the Middle East politics for the shocking price of gas. (It now costs ten times as much as it did when I started driving - and still we have it cheap. In Europe, gasoline costs twice what it does in the USA.) But the supply of petroleum is limited. The more cars use it, the faster everyone will run out.
Global warming has gotten ample attention in recent years, but hardly anyone talks about its link to overpopulation. We could live the same lifestyle that we now enjoy without overheating the planet if there were fewer of us. More people mean more cars, more electricity, more heat, and in many other ways more greenhouse gases.
Pollution has been slowed in recent years but it is still a problem and one that will resurface. Humans create various kinds of waste (trash, industrial byproducts, excreta) that the environment has to absorb. It can absorb and recycle these wastes up to a point. As we exceed that point by creating more waste, the environment's natural capacity for recycling these is exceeded, and the pollution builds up. As the population grows, the planet will become dirtier, more toxic.
Thus, my prediction is that in the next century, humanity will experience a crisis of nature against culture, one that is already building. Culture pushes for more population, while nature will have reached its limit and begin to rebel. (We are already seeing signs of this, but we do not acknowledge them as such.) The big question will be whether world culture can recognize the problem in time and turn its attention to reducing population.
I am not optimistic about this. Reducing population is extremely difficult for multiple reasons, especially when individual humans want to have children and often have them by accident even when they do not want them. A shrinking population imposes economic hardship on everyone in the culture. Moreover, even if individual countries here and there manage unilaterally to take steps to reduce their populations, they run the risk of being overrun by their neighbors.