Those in a long-term relationship who believed their partner had become less caring - when questioned after 10 years - said they responded less well to everyday stresses.
The team, led by the University of Edinburgh, also found the stresses recorded after a decade were linked to a higher risk of death in the subsequent decade.
Lead researcher Dr Sarah Stanton said: "The results suggest that if people have someone they can turn to - and whom they think supports them - then it can help them deal with the stresses of everyday living.
"This also has downstream associations with later health outcomes."
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Psychologists assessed a group of around 1200 people, aged 25 to 74, who had spouses or live-in partners.
They were assessed over a 20-year period starting between 1995 and 1996 as part of the National Survey of Midlife Development in the United States.
Those taking part were asked to rate how much their partner understood, cared about and appreciated them, and then responded to the same questions a decade later.
Researchers found people who reported a significant drop in their partner's responsiveness during the first decade of the study ran a 42 per cent higher risk of mortality in 2015 - 20 years after the first wave of testing.
The team says the link to a risk of earlier death is explained by those people experiencing more negative reactions to daily stresses.
