Imagine the second generation of 2,000 children pair and have children. By age 30 they have had 2,000 children and have no more. The island's birth rate remains 2.0 and its population is 6,000.

Imagine the third generation pair and have children and the first generation begins dying at the same rate a fourth generation is born. For each birth there is a death. For each death, there is a birth. The population of our imaginary island remains stable at 6,000.

Now, imagine an island similar to the one described in the first three paragraphs. However, on this island, people live 30 years longer than they live on the island described above. In other words, unlike the first island where the first generation is dying as the fourth generation is being born, on this second island, the first generation lives until a fifth generation is born. On such an island, population would stabilize at 8,000. In other words, as life spans increase, population stabilization occurs at a higher level.

In the U.S. increasing life span is not a major factor in our increasing population--today net immigration plays the major role. However, in some developing countries, improved sanitation, diets, and health care dramatically increase life spans which in turn lead, for a time, to increasing population. We want people to live longer, healthier lives. At the same time, we must understand the role of life span as we learn about population growth--as we Think Population!