Assimilation is possible only when the groups and individuals are in continuous contact with others. Hence isolation is a negation of assimilation. Not only physical isolation but even mental isolation retards assimilation.
(2) Physical or racial differences
Differences in physical appearance are often used as a means of discrimination. It is easy to keep some people apart on the basis of their skin color or other physical features. For example, we can see widespread discrimination between the Whites and the Negroes in almost all the places in the world.
(3) Cultural differences
If there are no common elements in the two cultures, the groups may remain apart socially even though they happen to stay together physically. They may even struggle for supremacy in their intermittent conflicts. Thus, wide cultural differences between groups in customs, religious beliefs, morals, values, languages come in the way of assimilation.
(4) Prejudice as a barrier to assimilation
Prejudice is the attitude on which segregation depends for its success. As long as the dominant group is prejudiced against a particular group which is kept apart assimilation cannot take place. Prejudice also hampers assimilation between constituent elements within a given society. Prejudice within a community, within a family, or within any group can only contribute to disunity and not to unity.
(5) Dominance and subordination
Dominance and subordination often come in the way of close intimate contact between groups. If the dominant group does not provide equal chances and opportunities for the minority or immigrant groups, assimilation is very slow to take place. Further, complete assimilation may not take place. Strong feeling of superiority and inferiority associated with dominance and subordination also retard the rate of assimilation.