If you didn't get a cell phone until your 20s, that's normal. Only takes a few years as a teen when your brain is still developing for that muscle memory to get hard-coded. Gen X right-handed males only had a few things to put in their pockets -- keys and maybe some mints/gum/chapstick. Makes sense to put them in your most accessible pocket and that muscle memory just sticks. Wallet was also always the right butt cheek because it needed to be reachable with the dominant hand. Younger folk might have had a phone before they had to worry about keys, or at least they realized at an early age that the phone is more important than the keys, so they might have had an easier time getting the muscle memory that the phone goes on the right side. I am hoping I can rewire my brain, but definitely going to accidently put my keys in with my phone more than a few times before I build that habit.