Marriage may make people more vulnerable to depression, according to a recent study.

Researchers from the University of Wisconsin-Madison found that people who experience chronic marital stress are less able to savor positive experiences, a hallmark of depression. They are also more likely to report other depressive symptoms.

"This is not an obvious consequence, if you will, of marital stress, but it's one I think is extraordinarily important because of the cascade of changes that may be associated," Richard Davidson, lead researcher of the study and founder of the Center for Investigating Healthy Minds at the University of Wisconsin-Madison's Waisman Center, said in a statement. "This is the signature of an emotional style that reveals vulnerability to depression."

Previous studies suggest married people are, in general, happier and healthier than single people. However, marriage can also be one of the most significant sources of long-lasting social stress. It's not all wedded bliss.

For the study, researchers recruited married adult participants to complete questionnaires rating their stress on a six-point scale. They were asked questions like how often they felt let down by their partner or how frequently their spouse criticized them. They were also evaluated for depression.

Roughly nine years later, the questionnaire and depression assessments were repeated. In year 11, the couples were invited to the laboratory to undergo emotional response testing, a means of measuring their resilience. Resilience, from an emotional perspective, reflects how quickly a person can recover from a negative experience.

The participants were shown 90 images, a mix of negative, neutral and positive photographs such as a smiling mother-daughter pair. The electrical activity of the corrugator supercilii, also known as the frowning muscle, was measured to assess the intensity and duration of their response.

Researchers said measuring how activated or relaxed the muscle becomes and how long it takes to reach the basal level again is a reliable way to measure emotional response and the tool has been used before to assess depression.

Study participants who reported higher marital stress had shorter-lived responses to positive images than those reporting more satisfaction in their unions. There was no significant difference in the timing of negative responses.

Researchers are now interested in how to help people change this weakened ability to enjoy positive experiences, to enable them become more resilient to stress.

The findings were recently published in the Journal of Psychophysiology.