2007, the share of total wealth owned by the top 1% of the population grew from 35% to 37%, and that owned by the top 20% of Americans grew from 86% to 88%. The Great Recession also caused a drop of 36% in median household wealth, but a drop of only 11% for the top 1%, further widening the gap between the top 1% and the bottom 99%. According to PolitiFact and other sources, in 2011, the 400 wealthiest Americans had more wealth than half of all Americans combined. 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. In 2013, wealth inequality in the U.S. was greater than in most developed countries, other than Switzerland and Denmark. In the United States, the use of offshore holdings is exceptionally small compared to Europe, where much of the wealth of the top percentiles is kept in offshore holdings. According to a 2014 Credit Suisse study, the ratio of wealth to household income is the highest it has been since the Great Depression. According to a paper published by the Federal Reserve in 1997, "For most households, pensions and Social Security are the most important sources of income during retirement, and the promised benefit stream constitutes a sizable fraction of household wealth" and "including pensions and Social Security in net worth makes the distribution more even". In Inequality for All—a 2013 documentary, narrated by Robert Reich, in which he argues that income inequality is the defining issue of the United States—Reich states that 95% of economic gains following the economic recovery which began in 2009 went to the top 1% of Americans (by net worth) (HNWI). A September 2017 study by the Federal Reserve reported that the top 1% owned 38.5% of the country's wealth in 2016. According to a June 2017 report by the Boston Consulting Group, around 70% of the nation's wealth will be in the hands of millionaires and billionaires by 2021. A 2019 study by economists Emmanuel Saez and Gabriel Zucman found that the average effective tax rate paid by the richest 400 families (0.003%) in the US was 23 percent, more than a percentage point lower than the 24.2 percent paid by the bottom