To begin with, recall that conventionalism is a relativist theory of ethics that maintains that what is good is determined relative to a society, convention, or culture. On this account, no society or culture is better than another.
A popular way to support conventionalism is to appeal to the cultural differences argument, which goes like this:
There are many examples where cultures have different moral values. For instance, some cultures value personal achievement and individual competition, whereas others value solidarity and cooperation.
In contrast to America, Japan also prefers cooperation over competition, but it does so in a different way. Family is highly valued in Japan, and from this comes the norm of “harmony” within the group, which means a respect for your role in society above pursuit of self-interest.
If disagreement between societies and cultures does indicate that moral values are relative, then the more evidence for disagreement we can find, the stronger the argument will be. More evidence can be found by looking back over history as well as looking at different parts of the globe.
An important part of this historical change is found in the fact that people used to justify slavery by saying that they were better than the people they enslaved. Part of the reason they did this was simply because the people they enslaved seemed different than the slave owners.
This is a typical case of associating what is unfamiliar, different, or “abnormal” with what is bad. In a similar way, the defenders of slavery associated the familiar or “normal” with the good when they rejected the idea that white people could be slaves.
These types of associations can be found in many different societies across history.
There are other ways that humans have unthinkingly preferred what they are familiar with over what is unfamiliar. For instance, victims can sometimes prefer the familiarity of their captors to the unfamiliarity of freedom (commonly referred to as Stockholm syndrome). This can be seen in broader contexts as well.