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Which form of intelligence begins to decline in middle adulthood gradually reducing the ability to engage in abstract reasoning?

According to research from the American Psychological Association, certain cognitive abilities that rely on processing speed and working memory begin to subtly decline around age 30. This decline is most noticeable in the form of intelligence that supports abstract reasoning and novel problem-solving, answering the question: Which form of intelligence begins to decline in middle adulthood gradually reducing the ability to engage in abstract reasoning?