The English grammar school system has widened the gap between rich and poor, according to a study published today.
Pupils who miss out on places at the country's 164 grammar schools are left at an "immediate disadvantage", it suggests.
Researchers from the University of Bristol, the University of Bath and the Institute of Education, University of London examined the pay of more than 2,500 people born between 1961 and 1983.
In areas with a grammar school system, the average hourly wage gap between the top 10% and bottom 10% of earners was £16.41 between 2009 and 2012.
But in similar areas that had gone comprehensive, the equivalent earnings gap was £12.33.
Professor Simon Burgess from the University of Bristol, who led the research, said the inequality could be explained by the calibre of teaching in both types of schools.
"Selective schooling systems sort pupils based on their ability and schools with high ability pupils are more likely to attract and retain high quality teaching staff," Prof Burgess said.
"This puts pupils who miss out on a grammar school place at an immediate disadvantage.
In total, 14 local authorities in England currently operate selective education systems.
For the study, a local education authority was defined as selective if more than 20% of its 13-year-olds were assigned their school place by selection.
The researchers analysed information gathered by Understanding Society, a study following the lives of people in 40,000 UK households.
This enabled them to take into consideration factors that could affect earning power, including gender, ethnicity, parents' education level and class and labour market conditions.
After allowing for such factors, the team found that 18% of the income gap between the highest and lowest earners could be explained by the school system.
High earners from grammar schools were also better off by £1.31 per hour on average than top earners born in similar comprehensive authorities.
At the other end of the scale, the lowest earners from areas with selective schools received significantly less than those in non-selective areas.
The gap at the bottom of the income scale was most evident among women, with the lowest-paid from grammar school areas earning £0.87 less per hour than women in comprehensive areas.
This may be because a disproportionate number of girls were assigned to secondary modern schools in the past, according to the researchers.
Average earnings in both types of area were found to be almost identical, with £8.59 per hour in selective areas to £8.61 in non-selective.
Selective Schooling Systems Increase Inequality, by Simon Burgess, Matt Dickson and Lindsey Macmillan, will be published today (May 29) as part of the IOE Department for Quantitative Social Science's working paper series.