One of the reasons many people favor groups of people like them may be that it reduces stress in their lives. This is a key consideration when trying to convince them to at least act altruistically.
Other people are typically the most stressful part of a person’s environment. The more predictable they are, the less stress the person will feel; and predictability is a natural consequence of sharing common characteristics, especially values and behavior. This is particularly important when it comes to key survival-related activities such as eating, sleeping, and mating. Ecologists have known for years that, in non-human species, interference with these necessary activities increases stress, which if not abated (by increased distance, for example) can lead to mass death. If this same mechanism is present in humans, and there is good reason to believe it is, then there will be a natural and healthy resistance to increasing the numbers and kinds of people competing for the resources associated with survival.
Humans are different in that we can regulate our behavior beyond just instinct. Rules and laws, when used responsibly and wisely, provide some additional protection against unhealthy amounts of competition (interference with each other); but when used irresponsibly or incompetently they can easily make things much worse. There is likely a natural limit to even how far this tactic can be used, which involves the availability and spatial distribution of resources (and people); until we approach that limit – which we may be doing already – I believe we should use regulation to its maximum benefit, enabling the survival of as many people as possible without increasing individual stress.