That's the point: the economists' age-old assumption that "equity" (fairness) and efficiency are in conflict – that more of one means less of the other – fits with their theories, but is now being contradicted by empirical studies, many of them coming from such authoritative institutions as the Organisation for Economic Co-operation and Development and the International Monetary Fund. Last year staff at the fund published a study finding that income inequality between households, as shown by an overall measure such as the "Gini coefficient" – which is zero when everyone has same income, rising to 1 when one person has all the income – adversely affects economic growth. Last week the fund's staff published a new study building on this analysis by looking at the experience of people in different positions at the bottom, middle and top of the distribution of incomes, in almost 100 advanced and developing countries over the 22 years to 2012. The new study confirms that a high Gini coefficient for net income (income earned in the market, less taxes and plus government cash benefits) is associated with lower growth in real gross domestic product over the medium term. But it also finds an inverse relationship between the size of the income share going to the rich (defined here as the top 20 per cent of households) and the speed at which the economy grows.

If the income share of the top 20 per cent increases by 1 percentage point, GDP growth is 0.08 percentage points lower in the following five years, suggesting that the benefits do not "trickle down" to the rest of us. By contrast, if the income share going to the poor (the bottom 20 per cent) increases by 1 percentage point, GDP growth is 0.38 percentage points higher in the following five years. This positive relationship between shares of disposable income and higher growth continues to hold for the second and third quintiles (blocks of 20 per cent) which, following American practice, the authors refer to as the middle class. (This must mean that people in the second top quintile are the upper middle.) Next time some economist assures you of all that, it will be safe to assume they're not keeping up with the research. The paper quotes other studies to help explain why higher income shares for the poor and middle class are growth-enhancing.

They note research showing that higher inequality lowers growth by depriving lower-income households of the ability to stay healthy and accumulate physical capital (a home, a car, a heating system) and human capital (education and training). "For instance, it can lead to underinvestment in education as poor children end up in lower-quality schools and are less able to go on to college," they say. "As a result, labour productivity could be lower than it would have been in a more equitable world." Other research finds that countries with higher levels of income inequality tend to have lower levels of mobility between generations, with parents' earnings being a more important determinant of children's earnings. As well, increasing concentration of income at the top could reduce total demand (spending), and so undermine growth, because the wealthy spend a lower fraction of their incomes than middle and lower-income groups do. "Extreme inequality may damage trust and social cohesion and thus is also associated with conflicts, which discourage investment," the authors say.

Inequality affects the economics of conflict as it may intensify the grievances felt by certain groups or reduce the opportunity cost of initiating and joining a violent conflict. If you're poor you've got less to lose. So what should governments that want faster economic growth be doing to promote it? "Redistribution through the tax and transfer [welfare benefits] system is found to be positively related to growth for most countries, and is negatively related to growth only for the most strongly redistributive countries," they say. "This suggests that the effect of stability could potentially outweigh any negative effects on growth through a dampening of incentives." The redistributive role of the budget "could be reinforced by greater reliance on wealth and property taxes, more progressive income taxation, removing opportunities for tax avoidance and evasion, and better targeting of social benefits while also minimising efficiency costs in terms of incentives to work and save".

"In addition, reducing tax expenditures [tax breaks] that benefit high-income groups most and removing tax relief – such as reduced taxation of capital gains, stock options and carried interest – would increase equity and allow a growth-enhancing cut in marginal labour income tax rates in some countries." Then there's the reform of the labour market. "Appropriately set minimum wages, spending on well-designed active labour market policies aimed at supporting job search and skill matching can be important." "Moreover, policies that reduce labour market dualism, such as gaps in employment protection between permanent and temporary workers – especially young workers and immigrants – can help to reduce inequality, while fostering greater market flexibility. "Labour market rules that are very weak or programs that are non-existent can leave problems of poor information, unequal power and inadequate risk management untreated, penalising the poor and the middle class,' they say. Sounds like our economists have a lot to learn.