The Whitehall II cohort study's 28-year follow-up found a link between social engagement and dementia and cognition.

With a 28-year follow-up, Association of social contact with dementia and cognition investigated the relationship between social interaction and incident dementia and cognitive because past studies had limited follow-up periods, exposing findings to reverse causation bias. 

This study's findings show that social interaction protects against dementia, and that more frequent contact imparts greater cognitive reserve, while it's conceivable that the capacity to sustain more social contact is a measure of cognitive reserve. Future intervention research should look at whether increasing the frequency of social contact is realistic, acceptable, and effective in modifying cognitive results.

= Here a link to the whole study