A very large database in the United States contains information about several thousand sibling pairs who have been followed since 1979. Families living in poverty, or even close to it, have been excluded. The parents in my sample also stayed together for at least the first seven years of the younger sibling's life. I will call this group the normals. These constraints produced a sample of 710 pairs.
Earned income is a good place to begin.
In 1993, when we took our most recent look at them, members of the sample were aged 28-36. The normals were in the middle, averaging 16,800. These differences are sizeable in themselves. They translate into even more drastic differences at the extremes. Suppose we take a salary of 50,000 or more as a sign that someone is an economic success.