Instinctual human behaviour. We're programmed to divide into groups of us or them. Being part of a group leads to benefits from that group, but, freeloaders cou

Instinctual human behaviour. We're programmed to divide into groups of us or them. Being part of a group leads to benefits from that group, but, freeloaders could just profess loyalty, get the benefits and move on. So we have an instinct to fight freeloaders by having genuine members of the group make a difficult to fake concession or sacrifice of some kind. Which could be something like genital mutilation, or, maintaining a clearly irrational belief at all times for your entire life, or, at all times treating the others like inhuman scum and having 0 tolerance for any of their ideas, or usually, a combination of such things.