Alzheimer's is twice as likely to affect single people as those living with a partner, according to research.
The study, from the Karolinska Institute in Stockholm, Sweden, showed regular social interaction, including keeping close to loved ones on a daily basis, can help keep the brain healthy.
People who live alone all their life have double the risk of Alzheimer's while those who get divorced in mid-life have triple the risk, it suggested.
The most affected group are people who are widowed before middle-age and who stay single thereafter. They are more than six times as likely to suffer Alzheimer's as people who are married by middle-age and stay married, the study suggested.
In contrast, married people and those living with a partner in mid-life have a 50% lower risk of Alzheimer's than those living alone.
About 700,000 people in the UK suffer from dementia, of whom around half have Alzheimer's.
Lead author Krister Hakansson said: "Living in a couple relationship is normally one of the most intense forms of social and intellectual stimulation.
"This study points to the beneficial effects of a married life, consistent with the general hypothesis of social stimulation as a protective factor against dementia."