For millions of older people, the hardest part of ageing may not be failing health or shrinking mobility. It may be the quiet feeling that nobody truly understands or needs them anymore.

A major international study led by the University of California, Davis, has found that loneliness — rather than simply living alone — may significantly increase the risk of mental decline and shorten life expectancy among older adults. The findings, published on June 15 in the Journal of Personality and Social Psychology, add fresh urgency to growing concerns about ageing populations across the world.

The research team analysed data from more than 175,000 adults aged over 50 across 18 countries, making it one of the largest studies to examine how social relationships affect ageing. Using advanced statistical models that tracked people through different stages of cognitive decline, researchers found that loneliness consistently predicted worsening brain function and earlier death.

“Loneliness is a perception," said Tomiko Yoneda, Assistant Professor of Psychology at UC Davis and the study's lead author. “You could be surrounded by a crowd of people and still feel lonely, whereas isolation is just being alone. Some people might be completely content in their solitude and not feel lonely.

That distinction proved crucial. While social isolation demonstrated a weak correlation with shorter lifespans and no consistent association with cognitive decline, loneliness emerged as a significant risk factor. A 10% increase in reported feelings of loneliness was linked to an 8%–9% higher risk of severe cognitive impairment, a condition that affects memory, thinking and daily functioning.

Researchers also discovered that people who frequently reported loneliness were 3% less likely to recover from mild cognitive impairment, an early stage of memory and thinking difficulties that sometimes improves. In practical terms, the study means lonely individuals may find it harder to regain lost mental abilities.

“This approach is crucial,” Yoneda said, “because cognitive impairment increases mortality risk, and both risks increase with age.”

The team of 24 researchers followed participants through different stages of cognitive health, examining how feelings of loneliness changed over time. Their findings suggest loneliness often appears before measurable cognitive decline becomes visible, making it a potential early warning sign.

“Loneliness may be most prominent in early stages of cognitive impairment but is also a risk factor after impairment develops,” said Eileen K. Graham, associate professor of medical social sciences at Northwestern University and supervising author of the study. “Lonelier individuals may be more likely to progress to more severe stages and less likely to recover.”

The findings align with previous research from organisations, including the World Health Organization and the U.S. National Institute on Ageing, which warned that chronic loneliness can increase stress hormones, disturb sleep, raise inflammation levels, and affect brain health.

Experts say the implications extend beyond individual suffering. Dementia care already places a heavy emotional and financial burden on families and healthcare systems. If loneliness accelerates decline, tackling it could become a public health priority. Researchers suggest routine loneliness screenings in hospitals and care homes, along with community programmes that help older adults build meaningful social connections.

As populations age worldwide, the study highlights a painful reality: growing old is not only about keeping the body healthy. It is also about preserving a sense of belonging. In an era of longer lives, that simple human need may prove as important as medicine itself.