Start with zero.

Subtract 10 if you smoke, four if you live too far from family members to make spontaneous visits, seven if you are often overwhelmed by stress, and six if you are a couch potato.

Add one, five, seven, or eight, respectively, if these descriptions do not apply to you. Divide this number by five and add it to 84 if you are male or 87 if you are female.

Excluding a few more factors, including genetics, alcohol intake and diet, this number is a good estimate of how long you can expect to live, according to the Life Expectancy Calculator, a featured part of a new book by two Harvard Medical School (HMS) professors.

Living to 100: Lessons in Living to Your Maximum Potential at Any Age by Dr. Thomas T. Perls, assistant professor of medicine, and Margery H. Silver clinical instructor in psychology, draws on their research with the New England Centenarian Study (NECS). Perls and Silver are the director and associate director of NECS, respectively.

The study is a joint HMS-Beth Israel Deaconess Medical Center project.

Through their research on centenarians--those over 100-in eight towns around Boston, Perls and Silver observed a number of common traits shared by people who have lived a century or more.

"For the most part, genes were the real secret," Perls said. "This idea that longevity runs in families is really true. We think that genes play a crucial role."

But though genes are important, Perls and Silver stressed that lifestyle and personality can add to or subtract from a person's lifespan.

"The vast majority of us probably have the genes to allow us to get to the mid to late 80s in excellent health, but these days we have average life expectancy in our 70s," Perls said.

Perls and Silver blamed smoking, drinking alcohol in excess, exercising too little and eating poorly among the reasons that people don't live as long as they could.

"There are many people who don't fulfill their longevity potential because they don't do the kind of things--live the kind of lifestyle--that are conducive to living that long," Silver said.

In contrast, most of the centenarians they studied had healthy lifestyles. None of them smoked, they avoided risky behaviors, and they stayed active.

"It's relatively simple. It's sensible eating, exercise, and learning to manage your stress," Silver said.

"The centenarians had an extremely good ability--compared to the general population--to handle stress and to managetheir emotions," Silver added.

Having strong social relationships and livingnear family was also a commonality among mostcentenarians.

Perls and Silver said they were especiallyamazed by how active many of the centenarianswere. Almost all of them were healthy and activeinto their 90s, and a significant portion werestill going strong--giving concerts or pursuingnew careers--even past the age of 100.

People need to get over the idea that growingolder automatically means illness and decreasedactivity, Perls said.

"I don't think the trick is staying young. Ithink the trick is aging well," Perls said