Personalities are typically thought to be genetically determined; not so, says the New Scientist: "We may learn our personalities, and adjust them to situations we find ourselves in over time." "Extroverts are born not made — or at least, that's what they say. But what if it's more subtle than that? What if we tailor our personalities to our surroundings to make the most of our genes? Conventional comparisons between identical and fraternal twins indicate that nearly half of individual differences in personality traits have some underlying genetic cause. So people have tended to think of personality traits as largely determined by genes, says evolutionary psychologist Aaron Lukaszewski of the University of California at Santa Barbara."