Experts in longevity science disagree on predictions for life-span in the 21st century. Will we continue to grow older or will life-span ... all » begin to shorten? Obesity and infectious diseases have been cited as potential limits, however some say we can expect life-span to continue to grow as it has. Dr. James Vaupel, of the Max Planck Institute for Demographic Research, talks with Stuart Rothenberg, of Roll Call newspaper, about research on negative senescence and the plasticity of longevity.