It certainly is not expressed very clearly. Let me explain what I understand it to mean. First of all, let's imagine that you were one of the eighteen-year-olds in the study, and let's also suppose that there was a completely even spread of best friends across the sample. That would mean that 3 different people named you as one of their best friends, and everyone else was also named by 3 different people as being one of their best friends. It would also tend to indicate that everyone would be earning the same sort of salary some years later.
Now let's suppose that you were particularly popular, and that 10 different people named you as one of their best friends. That's 7 extra "best" friends you have, and some years later you would be earning 7 x 2 = 14% more than the average person in the study.
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