"Humans, because of their intelligence, have found ways to [temporarily] defeat survival of the fittest. As areas get overpopulated, humans have moved to areas where they have a better chance of survival. Humans have found ways to increase food supply, through the use of fertilizers, pesticides, irrigation, and refrigeration, all of which require fossil fuels. They have developed trade, so that so areas with shortfalls can benefit from surpluses elsewhere. Humans have developed a world financial system, which has helped enable worldwide trade. The financial system has also allowed investors to pay for goods after they are put into service, so that the cash flow resulting from an investment can be used (after the fact) to pay for the cost of the investment. This enables investment, and faster use of resources, including energy resources. One of the reasons for continued upward population pressure is the fact that humans have evolved to live beyond their reproductive years. In their declining years, humans often need assistance, either from their offspring or from a public pension program, or both. Because of concern for their own old age, people without pensions tend to have enough children so that there is a significant chance that a child of the right sex will survive to adulthood. With improving medical care, this tends to lead to [an] ever-rising population."