I intented to say it is an inheritable characteristic, one which has been selected for because it confered a genetic advantage. Not for the individual, but for the gene, as it causes people to support their social and therefore genetic relatives at the expense of more genetically distant individuals. It's the trait that says 'Other tribes aren't people, so it's ok to slaughter them and steal their stuff to give to my own extended family.'
I'm sure the encoding is a lot more complicated than a simple 'shun the heretic' gene. Like much of human personality, it's going to be down to the exact interactions between hundreds of genes and their associated controlling DNA.
Taken in a modern context, it gives the ability to dismiss a whole class of people with ease. Like criminals. They get filed away in the 'not my tribe' box. Not real people. Probably a bunch of violent gansters, anyway. They must be scum, or they wouldn't be in prison, so let them suffer.