Cultures in which this was acceptable did not generally do it to members of their own tribe (in-group). It was only something to be done to rival tribe members

Cultures in which this was acceptable did not generally do it to members of their own tribe (in-group). It was only something to be done to rival tribe members (out-group) who were defeated in battle. It was an expression of domination and disrespect to opponents. (At least in Papua New Guinea - other regions YMMV)