Over the years, scientists have theorized that longer life spans among some animal species, including humans, might be attributed to the size of their brains, protective body features or even a superior ability to flee from predators.

Now, two UC Davis researchers propose that, among humans and other social species, a long life span is a desirable trait that has developed through the evolutionary process. In fact, their model of longevity suggests that long life spans among social species offer benefits conducive to even longer life spans in successive generations. Extension of the life span is a "self-reinforcing" process, they propose.

"This entirely reframes the way we consider the future of human life expectancy," says James Carey, a UC Davis authority on biology and the demography of aging and life span. "This model of longevity extension in humans, combined with evidence that human life span is not fixed but is continuing to increase in developed countries, suggests that there is every reason to believe that human life span will continue to increase in the foreseeable future."

Carey is a senior scholar at the Center for the Economics and Demography of Aging at UC Berkeley and one of four panelists who debated the future of human life span during the recent meeting of the International Union for the Scientific Study of Population. He and Debra Judge, a UC Davis research ecologist, published a paper titled "Life Span Extension in Humans Is Self-Reinforcing: A General Theory of Longevity" in a recent issue of the scientific journal "Population and Development Review."

Long life spans, they point out, offer numerous advantages to humans and other animals that live in social communities rather than individually. Small increases in longevity can be amplified in successive generations, leading to even greater longevity.

For example, a longer life allows more opportunity for humans and other species over time to develop the biological mechanisms that at older ages will maintain health and repair physical flaws. Increased longevity also makes it possible for older individuals to nurture and pass on resources to the younger generation. And, as the life span lengthens and experience increases efficiency, the products of the labor of the community can be reinvested among several generations, allowing individuals to specialize in their areas of strength, fostering innovations that, in turn, promote longer life.

Also, when the life span is extended, the number of offspring usually decreases, allowing parents to invest more resources for a longer period in children who are therefore more likely to survive, the researchers point out. This decreases the physical drain on the parents, resulting in adults and children that are healthier and longer lived.

Thus, long life breeds long life.

"This model of longevity suggests that continuing developments in science and technology, such as organ cloning, transplantation of organs from other animals and molecular medicine, will further equip humans to gain more control over their environment, health and overall quality of life," says Carey. "People increasingly will be able to make choices that will lead to longer lives for themselves and their children."

This research project was funded by the National Institute on Aging.