The child is regularly spanked.
The child frequently watches television.
If you're thoroughly flummoxed by now, Dubner said that the ECLS data only show correlations between different factors and children's test scores; they do not necessarily establish cause and effect. But there still are useful hints here about what matters in parenting.
"If you are smart, hard-working, well educated, well paid and married to someone equally fortunate, then your children are more likely to succeed," write Levitt and Dubner. "(Nor does it hurt, in all likelihood, to be honest, thoughtful, loving, and curious about the world.) But it isn't a matter of what you do as a parent; it's who you are."
There is more at www.freakonomics.com, and at Ned Potter's blog: http://abcnews.blogs.com/scienceandsociety/