Studies have shown that babies in wealthy families are exposed to more words and more complex words during development than babies in poor families. Maybe that alone would affect your verbal IQ tested at age 5.
This effect fades out as children get older:
http://economix.blogs.nytimes.com/2011/04/07/the-case-for-having-more-kids/April 7, 2011, 9:00 AM
The Case for Having More Kids
By DAVID LEONHARDT
Q. My sense of the research on nature and nurture is that both matter. On the one hand, genes clearly matter. On the other, young children of college graduates, for instance, know hundreds and hundreds more words on average than young children of high school dropouts. That difference is not mostly genetic.
You seem to have a different sense of the research. You write, �Adoption and twin research provides strong evidence that parents barely affect their children�s prospects.� What�s the brief version of how you try to persuade skeptics like me?
Mr. Caplan: The central idea of twin and adoption research is that disentangling nature from nurture is hard. Our intuition isn�t very helpful. Yes, kids of college-educated parents know more words. But why? Maybe their upbringing is the reason, as you suggest. But babies from college-educated families might excel even if raised by high school dropouts, by learning a higher fraction of the words they hear, or spending more time reading.
So what does the twin and adoption data say? Language fits a standard pattern. Consistent with your skepticism, upbringing has a noticeable effect on the vocabulary of young children. But as children mature, this effect largely fades away.
The Colorado Adoption Project found, for example, that 2-year-olds adopted by high-vocabulary parents had noticeably larger vocabularies. But as the kids grew up, their vocabulary scores looked more and more like their biological parents�. By age 12, the effect of enriched upbringing on vocabulary was barely visible.
