The findings come as a second study suggests that depression could be an early sign of the disease.

BBC: Clue to old age memory loss

Second, the study suggests that extremely optimistic people are less influenced by negative information because their brains process it less efficiently.

FORBES: Pollyanna Had Brain Damage! Study Claims Faulty Brain Wiring Sparks Unrealistic Optimism

This is due to the fact that in many parts of the world, the study suggests, women are at least 50% more likely to be exposed to second-hand smoke than men.

BBC: Passive smoking 'kills 600,000' worldwide