The first is a large-scale study from the U.K.’s University of Exeter and King’s College London building on previous research from the Lancet Commission on Dementia Prevention, Intervention and Care. The study followed 25,000 people ages 50 and older for several years. All had hearing difficulty and could use hearing amplification. One group got hearing aids, the other did not. All were given a battery of memory and cognitive, or thinking, tests.