According to CBO (and others), the precise reasons for the  rapid growth in income at the top are not well understood",: xi  but involved multiple, possibly conflicting, factors.: xi 

Causes include:

decline of labor unions – Unions weakened in part due to globalization and automation may account for one-third to more than one-half of the rise of inequality among men. Pressure on employers to increase wages and on lawmakers to enact worker-friendly measures declined. Rewards from productivity gains went to executives, investors and creditors. A study by Kristal and Cohen reported that rising wage inequality was driven more by declining unions and the fall in the real value of the minimum wage, with twice as much impact as technology. An alternative theory states that passthrough income's contribution is incorrectly attributed to capital rather than labor.
globalization – Low skilled American workers lost ground in the face of competition from low-wage workers in Asia and other "emerging" economies.
skill-biased technological change – Rapid progress in information technology increased the demand for skilled and educated workers.
superstars – Modern communication technologies often turn competition into a "winner take most" tournament in which the winner is richly rewarded, while the runners-up get far less.
financialization – In the 1990s stock market capitalization rose from 55% to 155% of Gross Domestic Product (GDP). Corporations began to shift executive compensation toward stock options, increasing incentives for managers to make decisions to increase share prices. Average annual CEO options increased from $500,000 to over $3 million. Stock comprised almost 50% of CEO compensation. Managers were incentivized to increase shareholder wealth rather than to improve long-term contracts with workers; between 2000 and 2007, nearly 75% of increased stock growth came at the cost of labor wages and salaries.
immigration of less-educated workers – Relatively high levels of immigration of low skilled workers since 1965 may have reduced wages for American-born high school dropouts;
college premium - Workers with college degrees traditionally earned more and faced a lower unemployment rate than others. Wealthy families are also more likely to send their children to schools which have large endowments, resulting in more grants and lower student debt. The cycle is completed when wealthier alums donate more and disproportionately increase the size of elite endowments. Elite colleges also have better access to financial expertise.
automation - The Bureau of Labor Statistics (BLS) found that increased automation had led to "an overall drop in the need for labor input. This would cause capital share to increase, relative to labor share, as machines replace some workers."
We haven't achieved the minimalist state that libertarians advocate. What we've achieved is a state too constrained to provide the public goods – investments in infrastructure, technology, and education – that would make for a vibrant economy and too weak to engage in the redistribution that is needed to create a fair society. But we have a state that is still large enough and distorted enough that it can provide a bounty of gifts to the wealthy.

—Joseph Stiglitz
policy – Krugman asserted that movement conservatives increased their influence over the Republican Party beginning in the 1970s. In the same era, it increased its political power. The result was less progressive tax laws, anti-labor policies, and slower expansion of the welfare state relative to other developed nations (e.g., the unique absence of universal healthcare). Further, variation in income inequality across developed countries indicate that policy has a significant influence on inequality; Japan, Sweden and France have income inequality around 1960 levels. The US was an early adopter of neoliberalism, whose focus on growth over equality spread to other countries over time.
corporatism and corpocracy – Excessive attention to the interests of corporations reduced scrutiny over compensation shifts.
female labor force participation – High earning households are more likely to be dual earner households.
stock ownership is tilted towards households at higher income and education levels, resulting in disparate investment income.
Higher income households are disproportionately likely to prosper when economic times are good, and to suffer losses during downturns. More of their income comes from relatively volatile capital income. For example, in 2011 the top 1% of income earners derived 37% of their income from labor, versus 62% for the middle quintile. The top 1% derived 58% of their income from capital as opposed to 4% for the middle quintile. Government transfers represented only 1% of the income of the top 1% but 25% for the middle quintile; the dollar amounts of these transfers tend to rise in recessions.

According to a 2018 report by the Organization of Economic Cooperation and Development (OECD), the US has higher income inequality and a larger percentage of low income workers than almost any other advanced nation because unemployed and at-risk workers get less support from the government and a weak collective bargaining system.

Instruction: Extract the causes of income inequality in the United States mentioned in the text. Separate them with a comma.
Response: The text lists the following as causes of income inequality in the United States: decline of labor unions, globalization, skill-biased technological change, superstars, financialization, immigration of less-educated workers, college premium, automation, public policy, corporatism and corpocracy, female labor force participation, and disproportionate stock ownership in households at higher income and education levels.