In older adults with mild cognitive impairment and no family history of Alzheimer's disease, bilingual patients convert to Alzheimer's disease significantly faster than monolingual patients, with a mean conversion time of 1.7 years for bilingual and 3.0 years for monolingual patients (p=0.002). However, among those with a family history, no significant difference in conversion time is observed between bilingual and monolingual patients.

Speaking two languages for many years makes the brain stronger and more connected. This lets the brain keep working normally even when harmful proteins start building up. So, by the time a person shows mild memory problems, they already have a lot of damage inside the brain. That damage then makes them go downhill faster once memory problems appear. If someone has a family history of Alzheimer's, the disease acts more aggressively from the start, so the extra brain strength from bilingualism is not enough to slow it down, and both groups decline at the same speed.