The 25 richest Americans, including Jeff Bezos, Warren Buffett and Elon Musk, paid a "true tax rate" of just 3.4% between 2014 and 2018, according to an investigation by ProPublica, despite their collective net worth rising by more than $400bn in the same period. The report by the non-profit news organization exposes the US tax system as income and wealth inequality continues to widen. ProPublica used Internal Revenue Service data to dive into the tax returns of some of America's wealthiest and most prominent people. It found that in 2007 Bezos, the founder of Amazon and already a billionaire, paid no federal taxes. In 2011, when he had a net worth of $18bn, he was again able to pay no federal taxes " and even received a $4,000 tax credit for his children. ProPublica created what it called a "true tax rate" for the wealthiest 25 Americans by comparing federal income tax paid between 2014 and 2018 to how their net worth increased on Forbes' well-regarded rich list over the same period. "The results are stark," ProPublica wrote. "According to Forbes, those 25 people saw their worth rise a collective $401bn from 2014 to 2018. "They paid a total of $13.6bn in federal income taxes in those five years, the IRS data shows. That's a staggering sum, but it amounts to a true tax rate of only 3.4%." By contrast, the median American household paid 14% in federal taxes. The top income tax rate is 37% on incomes over $523,600 for single filers.