"According to Free Trait Theory, we are born and culturally endowed with certain personality traits--introversion, for example--but we can and do act out of character in the service of "core personal projects." In other words, introverts are capable of acting like extroverts for the sake of work they consider important, people they love, or anything they value highly. Free Trait Theory explains why an introvert might throw his extroverted wife a surprise party or join the PTA at his daughter's school. It explains how it's possible for an extroverted scientist to behave with reserve in her laboratory, for an agreeable person to act hard-nosed during a business negotiation, and for a cantankerous uncle to treat his niece tenderly when he takes her out for ice cream."