I included this study in the second edition of my book, so I have the numbers handy. The study was conducted in 2008-2009. Average household income was $71,500

I included this study in the second edition of my book, so I have the numbers handy. The study was conducted in 2008-2009. Average household income was $71,500 USD. An annual income of $75,000 USD represented an income in the top third of US households during those years. (Source: http://www.pnas.org/content/107/38/16489.full http://www.pnas.org/content/107/38/16489.full)