When we find ourselves in a group, we might not always be aware of how the group effects how we think. Group Homogeneity Biases, and are great examples of how group identity can effect how we think and behave; and it suggests some reasons it can be hard to understand those who do not share our views.
As the name suggests this bias relates to the out-group, that is, people who are not members of the group (group members being are the 'in-group'). In it's simplest form it creates the bias that 'they are alike; we are diverse' that is to say that you (the in-group) are complicated, and diverse and nuanced and they (the out-group) are simple and all the same.
When you find yourself in a group, you will likely spend more time with its members. Exposed to the wide range of opinions and personalities that exist within the group; you may well a aware of a lively diversity. By contrast you will not have the same level of exposure to the out-group; and as such the views and opinions of the out-group will like be ungenerously represented, knowingly or otherwise, by in-group members. The lack of nuance will likely mean that the impression of the out-group becomes over-simplified and charactured; its worst features subtly, or quite possibly unsubtally, exadurated. The out-group become increasingly hard to understand, much less empathise with, makeing the in group all the more confident and united.
As C.S. Lewis puts it: -
There is always the danger that those who think alike should gravitate together into ‘coteries’ where they will henceforth encounter opposition only in the emasculated form of rumour that the outsiders say thus and thus. The absent are easily refuted, complacent dogmatism thrives, and differences of opinion are embittered by group hostility. Each group hears not the best, but the worst, that the other groups can say.
The In-group Homogeneity effect; is the tendency to assume that there is a greater level of homogeneity withing the In-group than there really is; especially when it comes to positive traits. The group wants to believe its members all share a common view and perspective and will be unlikely to spend time discussing what is presumed to be self-evident and agreed. This allows for divercity in such matters to go un-noticed. It's also esier for a group to remain strong and engage with more people if its foundation isn't to cloosely explored.