The Nation

That jolt of caffeine in a morning cup of coffee or tea improves an impulsive, extroverted person's work on complex reasoning tasks, but does the opposite for thoughtful introverts, two psychology professors said. Caffeine in doses equivalent to one to three cups of coffee helps both types of people perform simple mental jobs, the psychologists said they have found in research over seven years. William Revelle, a psychology professor at Northwestern University, and Kristen Anderson, an assistant professor at Colgate University, conducted both joint and separate studies on the effects of caffeine on thinking.

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