When people strongly identify with a group, they will also tend to strongly base their moral decisions on the norms of that group.
In this respect, group identity can dull moral sensibilities. At its worst, this sort of moral deference to group norms can take the form of “my country right or wrong” or “whatever is best for us is the right thing to do.”
When people do not strongly identify with a group but rather view themselves as autonomous individuals, they will tend to be more responsible and thorough when making moral decisions, assuming they are concerned with morality at all.
By providing general, ready-made answers to moral questions, groups remove the need for their members to think for themselves. Indeed, most groups stifle conversations and thoughts that go beyond group norms.
Most Americans, for example, do not question the sources of their news or the biases of the people presenting it to them. Similarly, most conversations in so-called polite society do not stray far from established values and interpretations.
When change happens in groups it usually comes from the top down or is due to a concerted efforts of single-issue activists. Both sorts of change reveal the hierarchical nature of virtually all groups. Top-down change is by definition hierarchical, while activist change generally always succeeds because it threatens a hierarchy or forces it to accept a new moral idea.
Gay marriage is an example of this phenomenon as activism caused the hierarchy of standard US moral culture to change and much of that change was also brought about by changes at the top of the hierarchy.
Of course, all people need groups. We learn from them and they support us in matters we don’t know much about. But groups also hinder us after we have learned what they have to teach us. This is especially true of large groups with many members who do not know each other personally.
Standard American culture, even with its many subgroups, is such a group. So is Christianity, academia, rural culture, etc. When we cede moral decision-making to the group(s) we identify with, we weaken our moral sense, and in weakening that we also weaken our intellectual and emotional responsiveness to the world around us.
In traditional Chinese Buddhism, most monks were expected to spend their formative years studying at one monastery until they were ordained at around the age of twenty. Then they were expected to travel alone or in pairs to see the world, teach, learn, and visit other monasteries. Sometimes they stayed for long periods of time in a particular monastery and sometimes they traveled for years, sojourning in a variety of temples. The underlying idea was to not become attached to a single group’s view of the world, but rather to explore and learn to rely on one’s own senses and sensibilities for the moral and intellectual decisions that lead to mental clarity and enlightenment.