By the age of six a third of children in Northern Ireland are identifying with one community, and 15 per cent are making sectarian comments, according to a study from the University of Ulster.
The study found that by the age of three Catholic children are twice as likely as Protestant children to say they do not like the police.
At that age they were also twice as likely to say they did not like Orange marches, while Protestant children of the same age were expressing a strong preference for the Union Jack over the Tricolour.
Over one in five showed some awareness of violence related to the conflict.
The study, "Too Young to Notice? The Cultural and Political Awareness of 3-6 Year Olds in Northern Ireland", was carried out by Dr Paul Connolly, Prof Alan Smith and Ms Berni Kelly of the University of Ulster.
It involved 352 children randomly selected from nursery and primary schools across Northern Ireland, representative of the community in terms of gender, age, religion, area and social class.
The interviews with the children typically involved showing them images such as Orange marches, Irish dancing, police Land Rovers, circles coloured orange, white and green or blue, white and red, and football shirts, and recording their comments.
They were also shown featureless pictures of people and told they were called stereotypical names like Sinéad or Craig, with professions or religious affiliations, and encouraged to talk about the people. The method used meant the children had the space to express themselves freely.
All their answers were then coded and statistically analysed.
The study showed that, while only a very small minority of three-year-olds made sectarian comments, this rose sharply during the first two years of compulsory schooling, which starts at five, so that by the age of six 15 per cent of children were making sectarian comments.
By this age the children were also showing strong preferences for certain first names, colour combinations and football shirts.
The children showed some awareness of cultural and political difference from an early age, with 51 per cent of three-year-olds showing a preference for certain events and symbols. However, they did not necessarily understand why they had such preferences, and only 5 per cent of Protestant children and 7 per cent of Catholic children identified with their own community at this age.
There were gender and social differences among the children showing sectarian attitudes, with boys much more likely to do so than girls, and children from working-class backgrounds more likely to express prejudice than middle-class children.
Among the most potent symbols of community identification were parades (significant for 49 per cent of three-year-olds) flags (38 per cent) and Irish dancing (31 per cent).
The authors point out that group membership and the holding of prejudiced attitudes are separate processes, and are dealt with in different chapters. However, both increased rapidly with the age of the children.
Three factors influenced the formation of community identification among the children - the family, the community and the school.
Dr Connolly said the fact that there is a rapid increase in community identification and in sectarian comments by the age of six, after two years of compulsory schooling "raises important questions about the indirect effects that our segregated school system is having on the development of young children's attitudes and awareness."
Only 4 per cent of children in Northern Ireland attend integrated schools.
The report says the overall figures are likely to underestimate the levels of sectarian attitudes among children.
Among the general recommendations in the report are that, from the age of three, children should be encouraged to explore and experience a range of different cultural practices, events and symbols; from about the age of five they should be encouraged to understand the negative effects of sectarian stereotyping; and, to help this process, nurseries and schools should work with parents and the local community, including, where possible, with the wider community.
The study was funded by the Community Relations Council in Northern Ireland in association with Channel 4 television, which has produced an award-winning drama for pre-school children based in Northern Ireland, Sarah and the Whammi.