Children who do not get into a grammar school will find themselves in "lower ability peer groups", which, in turn, "affects their chances of succeeding at school".
Professor Simon Burgess from the University of Bristol, who led the research, also said the inequality could be explained by the calibre of teaching in both types of schools.
– Professor Simon Burgess
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.
This puts pupils who miss out on a grammar school place at an immediate disadvantage.
In addition they will be part of lower ability peer groups, which also affects their chances of succeeding at school.