This article addresses the question of sorting pigs by weight or sex as they are placed into social groups at weaning, when entering the grow/finish phase, or as gestating sows. A contrast is made between the stable, non-aggressive social behaviour of pigs in a free-ranging environment and our concerns about aggression and dominance related problems in commercial production. It is hypothesized that dominance will only be a problem if resources are limited and their access can be controlled by the pigs. Thus, historical systems in which pigs were limit fed on the floor were subject to dominance related problems. Limit feeding continues with gestating sows and sorting remains necessary unless some form of individual feeding occurs. For grow/finish pigs, feed is available ad libitum and floor space is difficult to defend. Thus, sorting by weight does not appear to be important for this phase of production. Sorting based on nutritional requirements should be practiced, and this leads to split-sex pens in grow/finish,and sorting by weight in newly-weaned pigs.