1. Research on adult intelligence has shown that while some abilities decline with age, others are maintained or improve through life experiences and learning. 2. Two types of intelligence are identified: fluid intelligence involving short-term memory and abstract thinking tends to decline with age, while crystallized intelligence involving accumulated knowledge and vocabulary tends to increase with age. 3. Adults can maintain cognitive functioning through selective optimization, focusing on areas of expertise and compensating for losses, becoming more proficient in meaningful activities over time through experience.