"The final theory is the comparison theory, which holds that families are essentially comparison machines that greatly exaggerate even minor differences between siblings. Imagine, says McHale, two friendly children born in the same family. "One of those children is incredibly extroverted, and the other is just very sociable," says McHale. In the context of any other family, says McHale, the second child would be considered an extrovert. "But in this family," says McHale, "she’s the introvert."