QUOTE : Evidence for this theory comes from the finding that every species seems to have a genetically determined maximum lifespan. While the average life expectancy for humans has increased, the maximum lifespan has remained stable (approximately 120 years). There is also research to suggest that at an early age, our genes trigger hormonal changes in the brain and regulate the cellular reproduction and repair process. At some point in the process, the genes that promote growth are 'switched off,' and those that promote aging are turned on. This leads to a steady decline in the functioning of the body until death occurs. One may see changes not only in physical functioning, but in the cognitive, emotional, and psychological domains as well.