A University of Wales report of six local authorities in the south of the principality, shows that the covert selection methods widely believed to be used by many English secondaries are not operating across the border.
Researchers found that indicators such as poverty, poor literacy or special educational needs did not necessarily adversely affect children's chances of getting into their preferred school.
Dr Stephen Gorard, lecturer in research methods at the university's school of education, surveyed 80 schools educating 80,000 pupils from 1987 to 1997.
His findings show that market reforms had not led to greater social segregation between schools and that secondaries were now less discriminatory about their intake than before.
The study - which analysed admission trends in the six new unitary authorities of Cardiff, Swansea, Merthyr Tydfil, Bridgend, Rhondda Cynon Taff and Caerphilly - found that the proportion of children from socially-disadvantaged homes had increased in line with the expansion of popular schools.
Anecdotal and case evidence has suggested that it is not unknown for schools in London to hand-pick pupils following interviews with parents, during which they try to establish what car they own and where they went on holiday. Some heads also choose children depending on their address or post-code - also known as selection by mortgage.
The indications were that children whose parents had poor literacy, lacked confidence or the ability to "play the system" or did not own their own transport, would end up in "sink" schools.
Dr Gorard said: "It is clear that market forces have not led to the stratification that some evidence has suggested.
"One could argue that there are regional factors at play here, given that some schools are located in rural areas where there is no choice of school.
"However, I suspect that some of the anecdotal evidence is incorrect. It could be that schools are being as honourable and decent as they can within the limitations of their policies. In a sense these findings ask more questions than they answer."