Donald Hirsch, a consultant, writing in today's TES Scotland, stresses that comprehensive school systems produce narrower social differences than selective systems, based on analysis of the PISA survey of knowledge and skills of 15-year-olds across industrialised countries, carried out for the Organisation for Economic Co-operation and Development (OECD).
"The gap between the least and most socially advantaged students at the end of schooling tends to be wider than average in the countries that still divide pupils early in secondary education. Countries that divide students also have, on average, lower student performance," Mr Hirsch points out.
Top-performing countries such as Finland, Japan and Korea all have comprehensives that combine high achievement with small social differences. Hungary and Germany are below-average performers and separate pupils into different schools at the start of secondary education.
Mr McConnell is backing comprehensives but simultaneously proposing more diversity among schools and more ability groups in S1 and S2. Many headteachers counter that his policies would have a similar effect to streaming or selection and hold back the very pupils who are most disadvantaged or excluded. Others strongly support more setting as the best way to raise attainment across the ability range.
Mr Hirsch reveals that students from less advantaged backgrounds are more likely to go to worse-performing schools and underperform within each school, a finding that will chime with ministers' views about doing more for the least successful in Scottish schools.
"Yet this should not lead us to conclude - as so many UK commentators are prone to do - that 'our comprehensives have failed'. The PISA results show just how wrong it is to assume that working-class children in countries like Germany are more likely to achieve their potential because, if bright, they go to superior academic track schools, or, if less academic, they thrive in a different kind of school adapted to their needs," Mr Hirsch writes.
He adds: "In the face of such evidence, it would be hard to argue that dividing students at an early age into programmes suited to their needs is the way to reinvigorate our secondaries."
A TES Scotland survey in November found that the majority of secondary heads are far from convinced about the merits of setting as the answer to the problems in the first two years of secondary.
Returns from almost 70 heads across the country - one in six of the total - revealed a mixed picture. Overall, 62 schools have introduced setting in S1-S2 - 23 say that they want to do more setting; 20 have an increase under consideration; and 23 reject any increase, some on principle and others because they have already set in three or more subjects or because of staffing restrictions.
Platform, page 13