> "A twenty-year tracking study of 3,000 high school students demonstrated that only children have higher IQs than their peers with one sibling -- in fact, "the

> "A twenty-year tracking study of 3,000 high school students demonstrated that only children have higher IQs than their peers with one sibling -- in fact, "there are marked negative effects on IQ of increasing sib size." I didn't see any mention of household income in the NYT article. I'm interested to know if the researchers factored in household income because from what I've read, people with higher incomes tend to have higher IQs and fewer children, and intelligence is heritable.