Nature and Nurture Again

I want to comment on a common error in the discussion of my post Nature, Nuture, and Income.  (See for example the comments at Jane Galt, Kevin Drum and also the trackbacks).

Despite my warning, many people thought that graph was saying something important about the average income of adoptees versus that of biological children.  Hence many people argued that "the results" were explained by discrimination against Korean Americans, poor nutrition of the adoptees before being adopted, or low IQ of children given up for adoption.

For the record, Asian Americans in general and the children in this sample have higher income and more education than the average American.  More fundamentally, however, these comments have misunderstood what is to be explained.  It is true that the average income of the adoptees was lower than that of biological children but the adoptees are also younger.  Once you control for this and a few similar factors the mean income difference goes away (think of shifting the adoptee line up).  What remains, and this is the key point, is that the biological line is upward sloping and the adoptee line is flat.

What my post and the paper are all about is the difference in the slope of the two lines, i.e. why is it that child income increases with parental income for biological children but not for adopted children.

Addendum: Suppose we control for age and other factors which in effect will raise the adoptee line then we could phrase the results as 'adoptees do better than biological children raised in poor households but worse than biological children raised in rich households.'  The explanation is simple from the genetic point of view - adoptees are drawn more or less randomly while high income parents tend to pass on high-income genes and low income parents tend to pass on low-income genes.  It's going to be very difficult, however, to explain why poor parents treat their adopted children better than their biological children but rich parents treat their adopted children worse.