They are Close-knit

This group of people is close-knit, and they don’t believe in intermarrying with other people from other communities, no matter how pious or religious the other group may be. They are strictly forbidden from marrying other Christians and can only marry Amish people.

This being the case, they do sometimes run out of people to marry and end up marrying their second cousins. This sometimes results in some serious genetic mutations. However, marrying first cousins is strictly prohibited.