in laboratory settings, researchers have demonstrated that selected alterations in specific genes can extend lifespan quite substantially in yeast and roundworms, less so in fruit flies and less again in mice. some of the targeted genes have homologues across species and in some cases have been associated with human longevity. studies by becca levy, an associate professor of epidemiology and psychology at the yale school of public health, have found that positive beliefs about aging may also increase life span.