I don't think it's a myth, but haven't ever experienced it, despite keeping mixed groups together until 26ish weeks. Seems to me that traditional breeds, who free range, have space and stimulation, are rarely, if ever affected. Having visited a pig farm who routinely castrate (we had a demonstration) I think it's an unnecessary procedure to castrate every single male piglet, 'just in case'! It's not something I will be doing with my meat pigs.