If you were to examine the birth certificate of each soccer player in 200611’s World Cup tournament, you would most likely find a strange thing: excellent soccer players are more likely to have been born in the earlier months of the year than in the later months. Similarly, if you examined the European national youth teams that feed the World Cup and professional ranks, you would find this strange phenomenon to be even more noticeable. What might explain this strange phenomenon? Here are a few guesses: a) certainastrological(占星术的) signs bring superior soccer skills; b) winter-born babies tend to have higher oxygen capacity, which increases soccer staying power; c) soccer-mad parents are more likely to conceive children in springtime, at the annual peak of soccer craze; d) none of the above. Anders Ericsson, a 58-year-old psychology professor at Florida State University, says he believes strongly in “none of the above.” Ericsson grew up in Sweden, and studied nuclear engineering until he realized he would have more opportunity to conduct his own research if he switched to psychology. About 30 years ago, he conducted his first experiment on memory: training a person to hear and then repeat a random series of numbers. “With the firstsubject, after about 20 hours of training, his digit span had risen from 7 to 20,” Ericsson recalls. “He kept improving, and after about 200 hours of training he had risen to over 80 numbers.” This experiment and his later research indicated that memory itself is not determined by genes. Although people may exhibit inborn differences in their abilities to memorize, those differences are less important than how well each person “encodes” the information. And the best way to learn how to encode information meaningfully, Ericsson determined, was a process known as intentional practice. Intentional practice doesn’t mean simply repeating a task. It involves setting specific goals, obtaining immediate feedback and concentrating as much on technique as on result. Ericsson and his colleagues have studied different expert performers from various professions, including soccer. They found that expert performers are nearly always made, not born.15. What is the main idea of the passage?