Many social scientists have studied the causes of success. One American study claims that your social network is the key to success. Apparently, every extra friend that you have at school adds two percent to your salary later in life! Perhaps this gives you more self-confidence (自信心) or perhaps you have more people to support you. Another study links height and success: Every extra centimeter is worth another $300 per year. The trouble is that for every person who fits these theories, there is a special case.