Marriage vs. Alzheimer's

A Finnish study, reported in the British Medical Journal, reports that

Being widowed from mid-life onwards was associated with the highest risk of cognitive impairment later in life with a highly significant odds ratio of 7.67 for Alzheimer’s disease

Living without a partner for other reasons was also related to impaired cognitive functioning much later in life

They theorize that married people have someone to talk to in old age, which helps fend off Alzheimer's.