In 1858, William Farr, a physician working for the General Register Office in the British government, conducted an analysis of the health benefits of marriage. After reviewing the death statistics of French adults, Farr concluded that married people had a roughly 40 percent lower mortality rate than those in the “celibate” category. “Marriage is a healthy estate,” wrote Farr. “The single individual is more likely to be wrecked on his voyage than the lives joined together in matrimony.”

Farr was prescient. In recent decades, dozens of epidemiological studies have demonstrated that married people are significantly less likely to suffer from viral infections, migraines, mental illness, pneumonia and dementia. They have fewer surgeries, car accidents and heart attacks. One meta-analysis concluded that the health benefits of marriage are roughly equivalent to the gains achieved by quitting smoking. (These studies are primarily concerned with married couples, but there is little to reason to believe similar correlations don’t apply to any committed long-term relationship.) The effects are even more profound when the attachment is secure. For instance, one study of patients with congestive heart failure sorted their marriages into high and low quality brackets. The researchers concluded that marital quality was as predictive of survival as the severity of the illness, with people in poor marriages dying at much faster rate.

But if the correlations are clear, their underlying cause is not. Why does a good love affair lead to a longer life?

A new study in Psychological Science, by Richard Slatcher, Emre Selcuk and Anthony Ong, helps outline at least one of the biological pathways through which love influences our long-term health. The scientists used data from 1078 adults participating in the Midlife in the United States (MIDUS) project, a longitudinal study that has been following subjects since 1995. While MIDUS looks at many different aspects of middle age, Slatcher et al. focused on the correlation between the quality of long-term romantic relationships and cortisol, a hormone with far reaching effects on the body, especially during times of stress.

In particular, the scientists investigated changes in cortisol levels over the course of a day. Cortisol levels typically peak shortly after we wake up, and then decrease steadily as the hours pass, reaching a low point before bedtime. While nearly everyone exhibits this basic hormonal arc, the slope of the drop varies from person to person. Some people have steep slopes – they begin the day with higher initial levels of cortisol, leading to sharper declines during the day – while others have flatter slopes, characterized by lower cortisol levels in the morning and a smaller drop off before sleep. In general, flatter slopes have been associated with serious health problems, including diabetes, depression, heart disease and a higher risk of death.

What determines the shape of our cortisol slope? There’s suggestive evidence that social relationships play a role, with several studies showing a connection between interpersonal problems and flatter slopes, both among adults and young children. This led Slatcher and colleagues to look at measurements of perceived partner responsiveness among MIDUS subjects. (Such responsiveness is defined as the “extent to which people feel understood, cared for, and appreciated by their romantic partners.”) After collecting this relationship data at two time points, roughly a decade apart, the scientists were able to look for correlations with the cortisol profiles of subjects.

Here’s the punchline: more responsive partners led to steeper (and healthier) cortisol slopes. Furthermore, these changes in hormone production were triggered, at least in part, by a general decline in the amount of negative emotion experienced by the subjects. This study is only a first step, and it needs to be replicated with other populations, but it begins to define the virtuous cycle set in motion by loving relationships. When we have a more responsive partner, we get better at dealing with our most unpleasant feelings, which leads to lasting changes in the way we process cortisol. The end result is a longer life.

While reading this new paper, I couldn’t help but think about the pioneering work of Michael Meaney, a neuroscientist at McGill University. In the 1990s, Meaney began studying the link between the amount of licking and grooming experienced by rat pups and their subsequent performance on a variety of stress and intelligence tests. Again and again, he found that those pups who experienced high levels of licking and grooming were less scared in new cages and less aggressive with their peers. They released fewer stress hormones when handled. They solved mazes more quickly.

More recently, Meaney and colleagues have shown how these feelings of affection alter the rat brain. High-LG pups have fewer receptors for stress hormone and more receptors for the chemicals that attenuate the stress response; they show less activity in parts of the cortex, such as the amygdala, closely associated with fear and anxiety; even their DNA is read differently, as all that maternal care activates an epigenetic switch that protects rats against chronic stress.

A similar logic probably extends to human beings. The feeling of love is not just a source of pleasure. It’s also a kind of protection.

Slatcher, Richard B., Emre Selcuk, and Anthony D. Ong. "Perceived Partner Responsiveness Predicts Diurnal Cortisol Profiles 10 Years Later." Psychological Science (2015)