Some researchers have attributed this rise to the baby boomer generation. Historically, the baby boomers were the largest influx of children conceived after WW2. For this reason, Thomas Shapiro suggests that this generation "is in the midst of benefiting from the greatest inheritance of wealth in history". Inherited wealth may help explain why many Americans who have become rich may have had a "substantial head start". In September 2012, according to the Institute for Policy Studies, "over 60 percent" of the Forbes richest 400 Americans "grew up in substantial privilege", and often (but not always) received substantial inheritances.