Two-fifths of the population of developed countries have gained little over recent decades, driving a rise in income inequality that is damaging economic growth and that can only be reversed through a wide range of government measures, including higher taxes on the rich, the Organization for Economic Cooperation and Development said Thursday.

Seven years on from a landmark report that revealed a steady rise in inequality across its largely developed-country membership since the 1980s, the Paris-based research body identified the rise of what it termed “nonstandard” employment and a sharp reduction in the proportion of workers in the middle of the skills and incomes range as its main cause.

That contrasts with some other recent research on inequality, which has tended to focus heavily on a surge in the incomes of the top 1% of earners.

“In recent decades, as much as 40% of the population at the lower end of the distribution has benefited little from economic growth in many countries,” the OECD said. “In some cases, low earners have even seen their incomes fall in real terms. When such a large group in the population gains so little from economic growth, the social fabric frays and trust in institutions is weakened.”

The OECD said the rise in inequality in the decades running up to the 2008 financial crisis has continued in its wake. While in the 1980s the top 10% of earners had incomes that were seven times as large as those of the bottom 10%, that ratio had risen to 9.6 across the OECD’s 34 members by 2013. In The U.S., that ratio rose from 15.1 in 2007 to 18.8 in 2014, a level of inequality only surpassed by Mexico.

Over the course of three major reports on inequality and its implications, the OECD has become more convinced that its rise not only threatens social cohesion, but also economic growth, which has been unusually weak across many major economies over the last decade, a fact that increasingly worries and puzzles economists and policy makers. It estimates that the rise of income inequality between 1985 and 2005 knocked 4.7 percentage points off cumulative growth between 1990 and 2010.

“We have reached a tipping point,” said OECD Secretary-General Ángel Gurría. “Inequality in OECD countries is at its highest since records began. The evidence shows that high inequality is bad for growth. The case for policy action is as much economic as social. By not addressing inequality, governments are cutting into the social fabric of their countries and hurting their long-term economic growth.”

The OECD’s main role is to provide policy advice to its 34 member governments, which include that of the U.S. While its suggestions aren’t always followed by its members, they do offer a benchmark by which to measure the commitment of those governments to addressing identified problems.

Its recommendations for reversing the damaging rise in inequality include a wide range of measures, from easing the obstacles that make it difficult for mothers to work, to boosting educational attainment and skills acquisition among the children of workers at the lower end of the income distribution.

But the fostering of better jobs for a larger proportion of the workforce will be key to reducing inequality, the OECD said.

“The focus must be on policies for quantity and quality of jobs; jobs that offer career and investment possibilities; jobs that are steppingstones rather than dead ends,” it said.

The narrowing, but still wide gap, between the incomes of men and women is one source of inequality the OECD believes can be reduced relatively easily.

“It looks like the lowest hanging fruit. If you make it easier for women to work, and close those gender pay gaps, you reduce inequality and improve growth,” said Mark Pearson, the OECD’s deputy director of Employment, Labour and Social Affairs.

Progress on other fronts may be more difficult. The report marks a step forward for the OECD in recognizing that the destruction of “skilled, but routine” jobs as a result of technological change and globalization has been a major driver of the rise in inequality. In its 2011 report on the causes of rising inequality, the OECD exculpated globalization, and continues to see it as a factor that accelerates the inevitable impact of technological change, rather than being a root cause in itself.

“Because the rise in inequality is so deeply embedded in our economic structures, it will be hard to reverse it,” the OECD said. “Changing institutions, policies, and relationships between economic actors that have been with us for so long will be far from easy. And, forces of technological change and globalization are not going away.”

Those forces have resulted in a decline in traditional, full-time employment. Temporary and part-time work, and self-employment now account for about a third of total employment in OECD countries. Since the mid-1990s, more than half of all job creation was in the form of “nonstandard” work.

“Many nonstandard workers are worse off in many aspects of job quality, such as earnings, job security or access to training,” the OECD said. “In particular low-skilled temporary workers face substantial wage penalties, earnings instability and slower wage growth.”

The new report dismisses the idea that greater equality of opportunity—for example, equal access to high levels of education—is possible without a prior reduction in inequality of incomes.

“What we show is that it is very difficult to imagine that you can have equality of opportunity if you have large inequalities of income,” said Mr. Pearson. “If you reject equality of outcome as a goal in favor of equality of opportunity, you’re going to fail.”

The OECD said higher taxes on the wealthy to provide financial help to the less well-paid would “not necessarily harm growth.”

“Redistribution via taxes and transfers is a powerful instrument to contribute to more equality and more growth,” it said.

But it said there is a limit to what can be accomplished through fiscal measures.

“The most efficient policy package will address inequalities at the point where they originate rather than trying to pick them up only at a later stage,” the OECD said.