First emerged in the middle ages due to the land bit as mentioned above and added security due to the lawlessness of Scotland (for a change..). The Border Families started in a similar lawless environment due to the Wars of Independence that made it difficult to raise crops, graze your cattle and sleep well at night. Since the borderers couldn't expect protection by central government, they took matters into their own hands, developed their own code of (mis)conduct followed a 'Heidman' (Read Chieftain) and went about their business of 'Reiving'. This, incidently, was as much an English occupation as a Scots one, the Charlton's, Robson's and Storey's no less tribal than the Scott's, Elliott's and Johnstone's.

Fave Reiver quote:
An English traveller was making his way through Liddesdale (brave/idiotic) and was shocked by the lack of churches to be seen around, he asked a local 'Are there no christian's here!?' the Liddesdale man grinned and replied 'Naw were a' Armstrangs an' Elliotts!'