The study, published in Journals of Gerontology, followed 6,677 adults for just under seven years.
The quality of a person's social circle appeared more important than the overall size, the research team said.
The Alzheimer's Society said it was essential to help patients to maintain "meaningful social connections".
None of the participants had dementia at the start of the trial, but 220 were diagnosed during it.
The research group compared the traits of those who did and did not develop dementia to find clues as to how social lives affect risk.
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