R. M. Hare’s classic example of supervenience is that if one room is nice, then another room that is the same in all other respects is also nice. The niceness of the rooms supervenes on their other characteristics. This is clearly right. However, it is also true that if one room is nice, then another room is nice provided it is the same in all respects except that there are testimonials to its niceness. Niceness does not supervene on testimonials to niceness.
Most philosophers believe that membership in the moral community is a supervenient property. Here, it is maintained that it is a matter of testimonials and that it is not the case that value supervenes on the testimonials.
Here is how it works. People naturally believe that they are of value, full stop. They do not naturally believe that their value supervenes, let alone that it supervenes on particular properties they possess. They effectively provide testimonials to their own value and have a naturally selected disposition to accept the testimonials of others. Once someone accepts someone else’s testimonial to their own value, they are committed to accept the testimonials of everyone else who gives a testimonial to their own value. Thus, people who believe they are of value are committed to the existence of a moral community that includes everyone who believes they are of value.
Since people effectively affirm that that they are of value but not that their value supervenes, they effectively affirm that they are of value throughout their existence, no matter how they change. Therefore, although infants do not give testimonials to their own value, it is reasonable to believe that they are of value because they will grow up to give testimonials. They are of value throughout their entire existence but they do not have to give testimonials throughout their entire existence to be acknowledged to be of value.