By Lindsay Peterson

We think of aging as something we do alone, the changes unfolding according to each person's own traits and experiences. But researchers are learning that as we age in relationships, we change biologically to become more like our partners than we were in the beginning.

"Aging is something that couples do together," says Shannon Mejia, a postdoctoral research fellow involved in relationship research at the University of Michigan in Ann Arbor. "You're in an environment together, and you're appraising that environment together, and making decisions together." And through that process, you become linked physically, not just emotionally.

It's like finishing each other's sentences, but it's your muscles and cells that are operating in sync.

Doctors tend to treat people as individuals, guided by the need to ensure patient confidentiality. But knowing about one partner's health can provide key clues about the other's. For instance, signs of muscle weakening or kidney trouble in one may indicate similar problems for the other.

Looking at married couples who were together less than 20 years and couples together for more than 50, Mejia and her colleagues have found striking similarities between partners who have spent decades together, especially in kidney function, total cholesterol levels and the strength of their grips, which is a key predictor of mortality.

The data came from 1,568 older married couples across the United States. The couples were part of a larger dataset that included information on their income and wealth, employment, family connections and health, including information based on blood tests.

One obvious reason for partner similarity is that people often choose partners who are like them — people from the same stock, with similar backgrounds. But that didn't explain why there were more similarities between the long-time partners, compared to the others.

To learn more about this element of partner choice versus spending decades together, the researchers analyzed couples by age, education and race. When they accounted for the effect of partner choice, they found that the biological similarities persisted, based on markers in blood tests.

The way Mejia puts it, this likeness includes "something the couples co-created" over time, not just what they started with because they were similar at the beginning.

She's now studying what may be causing these "co-created" biological similarities. "We're working on a few things," she said, such as the effect of partners' shared experiences and of sharing an environment where they have similar advantages and disadvantages, like the ability to walk in their neighborhoods or find other ways to stay active.

Mejia's work follows that of Christiane Hoppmann, an associate professor at the University of British Columbia, in Vancouver. She and her colleagues found that longtime couples experienced similar levels of difficulty with daily tasks, such as shopping for food, making a hot meal and taking medications. They found the same for depression, and with both depression and daily task difficulties, they found that the couples changed, for better or for worse, in sync.

They also found that the effects crossed over from the mental to the physical. In other words, increases in feelings of depression in one spouse led to more daily task limitations in the other.

Hoppmann and Denis Gerstorf, of Humboldt University in Berlin, suggest that a key factor here could be physical activity. For instance, if a depressed partner refuses to leave the house, the other may feel compelled to remain at home, too. The longer the two remain sedentary, the more vulnerable they become to a range of problems, from worsening depression to diabetes, that can limit their ability to function from day to day.

But the news in these partner studies is not all bad.

William Chopik, an assistant professor of psychology at Michigan State University, has found evidence of the power of optimism. He and his research colleagues studied optimism, in addition to health and activity limitations, in 2,758 older couples in a national dataset. Optimism scores came from a test that measured their level of agreement or disagreement with statements such as "in uncertain times, I usually expect the best."

The researchers found that over a four-year period, when one partner's optimism increased, the other partner experienced fewer illnesses such as diabetes and arthritis compared to people whose partners did not become more optimistic. So, "the fact that (your spouse) increased in optimism is good for you," even if your optimism didn't rise, Chopik said.

He and his colleagues had accounted for age, gender and education differences. He speculates that optimists are more likely to live healthy lives and use their influence over their partners to get them to live healthier, too.

Chopik is currently studying how two partners' levels of cortisol, a hormone related to stress, change and become coordinated over time. He plans to compare couples whose relationships span at least 40 years to those who have been together for less than two.

These investigations of how couples affect each other's health are relatively new, particularly the research into the biological changes, and the researchers are still searching for explanations.

Nevertheless, they say, the implications for health care are clear. People in relationships don't experience chronic health problems on their own. When a spouse comes in with a problem, the other spouse could be part of the cause — or the solution.